Car Repair Glossary

The two halves of a V6 or V8, each with its own cylinder head and its own oxygen sensors. Bank 1 is the one containing cylinder 1, and which side of the car that is changes from engine to engine, so it's found by locating cylinder 1 rather than by guessing at the passenger side. An inline engine has one bank and can't set a bank 2 code at all.

Where this comes up: P0430 Code: Bank 2 Location and What Actually Fails · Catalytic Converter Replacement Cost: When It's $0 · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · P0011 Code: Check the Oil Before You Buy a Solenoid · and 6 other articles

The first minutes after the engine has sat long enough to cool all the way down, usually overnight. Several self-tests only run then, and several faults only appear then, which is why a shop asks when the light came on and why a test drive from a warm engine can miss the fault entirely.

Where this comes up: P0456 Code: Will It Clear Itself, and What It Costs · 0W-20 Substitute: 5W-20 and Almost Nothing Else · 0W-20 vs 5W-30: What Happens If You Use the Wrong One · 22 High-Mileage Cars to Avoid: Ranked Worst First · and 24 other articles

The particular mix of driving the self-tests need before they will run: a cold start, some idling, a steady cruise, and enough time at each. It's why a car that has just been repaired may still show not ready, and why one trip around the block proves nothing.

Where this comes up: P0456 Code: Will It Clear Itself, and What It Costs · Do Catalytic Converter Cleaners Work? Two Cases of Six · Honda CR-V 2017-2022 Oil Dilution: The 1.5T Problem · P0011 Code: Check the Oil Before You Buy a Solenoid · and 3 other articles

The state inspection that reads the computer rather than the exhaust on most modern cars. A stored code fails it on its own, and so does a car whose readiness monitors have been reset, which is why clearing a code the day before the test is the worst possible timing.

Where this comes up: P0128 Code: Thermostat, Sensor or a Software Update? · ABS Light On: What It Means and Is It Safe to Drive? · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Catalytic Converter Replacement Cost: When It's $0 · and 16 other articles

A snapshot of what the engine was doing at the instant a fault was stored: speed, load, coolant temperature, fuel trim. It's the difference between knowing a code exists and knowing when it happens, and it's free to read at the same time as the code.

Where this comes up: Check Engine Light Codes: 26 OBD-II Codes, 6 Families · Check Engine Light Meaning: The Test Your Car Failed Twice · P0011 Code: Check the Oil Before You Buy a Solenoid · P0299 Code: Turbo Underboost Meaning, Causes and Fix · and 5 other articles

The correction the engine computer makes to the fuel it delivers, and the number behind most lean and rich codes. Short term trim moves constantly; long term trim is what the computer has learned it must keep doing. When long term trim runs out of room, the code sets.

Where this comes up: P0171 Code: System Too Lean Bank 1, Causes and Fixes · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · Check Engine Light Meaning: The Test Your Car Failed Twice · and 6 other articles

Too much air for the fuel, or too much fuel for the air. Lean burns hotter and is the one that damages parts; rich wastes fuel and loads the catalytic converter with what didn't burn. Neither names a part on its own.

Where this comes up: P0171 Code: System Too Lean Bank 1, Causes and Fixes · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · P0101 Code: The Sensor Is Often Telling the Truth · P0172 Code Meaning: Too Much Fuel and How to Fix It · and 4 other articles

A protective state where the car locks itself into one gear, or caps power, after reading something it doesn't trust. It feels like a catastrophic failure and usually isn't: it's the car working, and the cause is often a sensor rather than the mechanism it was protecting.

Where this comes up: Bad Transmission Symptoms: Is Your Transmission Going Out? · Blown Head Gasket Symptoms: The Signs and How to Test · CVT Transmission Problems: It Depends Whose CVT · Car Shuts Off While Idling or Driving? Causes and What to Do · and 6 other articles

The check engine light's real name, and the one every manufacturer bulletin uses. Steady means a fault is stored; flashing means fuel is reaching the exhaust unburned, which is the one case where the advice is to stop driving.

Where this comes up: P0700 Code: The One That Points at Another Code · Check Engine Light Meaning: The Test Your Car Failed Twice · Flashing Check Engine Light: $256 Turning Into $1,511 · Low Transmission Fluid Symptoms and When to Stop Driving · and 6 other articles

The diagnostic system every car sold in the US has had since 1996, and the reason any scan tool fits any car. It's why a code has the same meaning on a Ford and on a Toyota, and why a $20 dongle can read what a dealer reads. What it doesn't standardize is the repair: the code says what the computer measured, not which part to buy.

Where this comes up: Check Engine Light Codes: 26 OBD-II Codes, 6 Families · ABS Light On: What It Means and Is It Safe to Drive? · Blown Head Gasket Symptoms: The Signs and How to Test · Check Engine Light Meaning: The Test Your Car Failed Twice · and 25 other articles

A fault the car has seen once and isn't yet willing to light the lamp for. Most emissions faults have to appear on two separate drives before the check engine light comes on, and the pending code is the record of the first one.

Where this comes up: Check Engine Light Meaning: The Test Your Car Failed Twice · P0455 Code: Large EVAP Leak Cost, Starting at the Cap · P0456 Code: Will It Clear Itself, and What It Costs

The self-tests the computer runs on each emissions system, and the thing an inspection station reads before it reads anything else. Clearing a code resets them to not ready, which fails the test just as surely as the code would have, and they only reset themselves by being driven.

Where this comes up: Check Engine Light Meaning: The Test Your Car Failed Twice · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · Dashboard Warning Lights: Three Mean Stop, Five Can Wait · P0128 Code: Thermostat, Sensor or a Software Update? · and 7 other articles

The device that reads what the engine computer has stored, from a $20 dongle to a workshop tablet. The cheap ones read the code and clear it; the ones worth having also show live readings and the freeze frame, which is where the useful part of a diagnosis sits.

Where this comes up: Check Engine Light Meaning: The Test Your Car Failed Twice · ABS Light On: What It Means and Is It Safe to Drive? · ABS Sensor: Wheel Speed Sensor Symptoms, Location, Cost · Bad Transmission Symptoms: Is Your Transmission Going Out? · and 34 other articles

A fault the computer has decided is real and written down, which is what turns the light on. It stays stored until it is cleared or until the car passes the same test enough times to forget it, and it's the thing an emissions station reads.

Where this comes up: P0174 Code: System Too Lean Bank 2, Causes and Fixes · ABS Sensor: Wheel Speed Sensor Symptoms, Location, Cost · CVT Transmission Problems: It Depends Whose CVT · Car Jerks When Accelerating: Causes, Costs and Fixes · and 20 other articles

Which side of the catalytic converter a sensor sits on. The upstream sensor measures what the engine sent out and is what fuel trim corrects from; the downstream one exists to watch the converter, and when the two start reading alike the computer decides the converter has stopped working.

Where this comes up: P0420 Code: Bank 1 Meaning, Causes and Repair Cost · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Catalytic Converter Replacement Cost: When It's $0 · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · and 10 other articles

A deliberate series of firm stops that lays an even film of pad material onto the disc. Done properly it is what makes brakes quiet and consistent; skipped, the film lands unevenly and produces the pulse people call warping.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills · Car Smells Like Burning: What Each One Costs, When to Stop · Do High-Mileage Cars Burn Oil? Is a Quart per 1,000 Normal · Knocking Noise When Braking: 8 Causes and Is It Safe?

A surface grinding process that can remove the hard spots an ordinary brake lathe cuts around. It's the only way to save a disc that has gone to cementite, and by the time you need it a new disc almost always costs less than finding a shop that offers it.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills

The clamp that presses the pads against the disc, driven by hydraulic pressure from the pedal. A seized caliper drags one wheel, which shows up as heat, smell and uneven pad wear long before it shows up as a noise.

Where this comes up: Brake Fluid Leak Repair Cost: Caliper or Master Cylinder · Bad Wheel Bearing Symptoms and How Long You Can Drive · Brake Pad Replacement Cost: With or Without Rotors? · Car Fluid Leak Repair Cost: Which Ones Can Wait · and 6 other articles

A hard, abrasive form of iron carbide that cast iron transforms into above roughly 650 °C. It forms under a hot spot on the disc, and it can't be polished, driven or machined out, because a lathe cuts around it. A disc with cementite is finished.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills

A brake disc that is slightly thicker in some places around its circumference than others. Each thick patch passing between the pads pushes them apart, and that movement travels back up the brake fluid as a pulse in the pedal. It's what almost every complaint of a warped rotor turns out to be.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills · Resurface or Replace Brake Rotors? One Measurement · Steering Wheel Shakes When Braking: 5 Causes by Cost

The dimension at which a disc must come off and be scrapped, machined or not. It sits only about 0.015 in. below the machine-to figure, so the working margin between can be cut and must be replaced is very small.

Where this comes up: Resurface or Replace Brake Rotors? One Measurement

The flat machined surface the brake disc bolts against. It corrodes on any car that has seen winters, and a disc fitted over that corrosion sits very slightly crooked. Whether a shop cleans it is the single thing that decides if a brake job lasts or comes back.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills · Brake Pad Replacement Cost: With or Without Rotors? · Steering Wheel Shakes When Braking: 5 Causes by Cost

Side-to-side wobble of the disc as it turns, measured with a dial indicator on a fitted disc. The limit on most vehicles is 0.002 in. Runout doesn't cause a pulse directly. It causes the pads to wear one band of the disc thinner, which then does.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills

The thinnest a disc may be machined to and still go back on the car. It's cast or stamped into the disc itself, usually near the hat or the outer edge. A disc already below it can't be resurfaced at any price.

Where this comes up: Resurface or Replace Brake Rotors? One Measurement · Brake Pad Replacement Cost: With or Without Rotors? · Head Gasket Replacement Cost and Repair: Is It Worth Fixing? · Steering Wheel Shakes When Braking: 5 Causes by Cost · and 1 other article

A micrometer with one flat anvil and one pointed anvil. The point reaches into the grooves worn by the pads; ordinary flat jaws bridge across them and report a thickness the disc doesn't have anywhere.

Where this comes up: Resurface or Replace Brake Rotors? One Measurement

The thin film of pad material that normally coats a brake disc. Brakes are designed to work through it, so it's not contamination. The trouble starts only when it's laid down unevenly.

Where this comes up: Warped Rotors: One Word, Three Faults, Three Bills · Knocking Noise When Braking: 8 Causes and Is It Safe? · Resurface or Replace Brake Rotors? One Measurement · Steering Wheel Shakes When Braking: 5 Causes by Cost

The letter code in the round API mark showing which performance standard the oil meets. Later letters supersede earlier ones, so an engine asking for an older category is generally satisfied by a newer oil. The reverse isn't true.

Where this comes up: How to Read an Oil Bottle: API, ILSAC, OEM Approvals · 0W-20 Substitute: 5W-20 and Almost Nothing Else · 0W-20 vs 5W-30: What Happens If You Use the Wrong One · Can You Mix Synthetic and Conventional Oil? Two Questions · and 2 other articles

API's five-way classification of the stock an oil is built from. Groups I and II are refined crude; Group III is heavily processed crude that is still sold as synthetic in the US; Group IV is built molecule by molecule. Two bottles both marked synthetic can be different groups.

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General Motors' own oil specification, stricter than the API category on several counts including volatility. A manual that asks for dexos has excluded conventional oil without using the word synthetic anywhere.

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High-temperature high-shear viscosity: how thick the oil still is at 150 °C while being sheared between moving parts. It's the number that describes the film actually protecting a bearing, which is why thin grades are defined by it.

Where this comes up: 0W-20 Substitute: 5W-20 and Almost Nothing Else · 0W-20 vs 5W-30: What Happens If You Use the Wrong One · How to Read an Oil Bottle: API, ILSAC, OEM Approvals · What Is Oil Dilution? Fuel in Your Oil, and the Limits

The starburst standard covering fuel economy and engine protection for gasoline engines. GF-7A and GF-7B replaced GF-6A and GF-6B on March 31, 2025. The B versions are for the very thin 16-grade oils and aren't interchangeable with anything else.

Where this comes up: How to Read an Oil Bottle: API, ILSAC, OEM Approvals · 0W-20 Substitute: 5W-20 and Almost Nothing Else · 0W-20 vs 5W-30: What Happens If You Use the Wrong One · Can You Mix Synthetic and Conventional Oil? Two Questions · and 2 other articles

How much oil evaporates in an hour at 250 °C, as a percentage of weight. It's the mechanism behind most cars that seem to drink oil between changes, and the specification that quietly rules conventional oil out of many modern engines.

Where this comes up: Synthetic vs Conventional Oil: Most "Synthetic" Is Crude

Unburned fuel washing past the rings and collecting in the oil pan, thinning the oil and raising the level on the dipstick. Short cold trips are the classic cause. Laboratories generally treat about 4% fuel in oil as excessive.

Where this comes up: What Is Oil Dilution? Fuel in Your Oil, and the Limits · 22 High-Mileage Cars to Avoid: Ranked Worst First · Do High-Mileage Cars Burn Oil? Is a Quart per 1,000 Normal · Ford 1.0 EcoBoost Wet Belt: Is Your Car in the Recall? · and 8 other articles

An exhaust component that converts carbon monoxide, unburned fuel and nitrogen oxides into less harmful gases, using platinum group metals as the catalyst. Those metals are why it is expensive, and why it is worth stealing. It rarely fails on its own: something upstream usually kills it.

Where this comes up: P0420 Code: Bank 1 Meaning, Causes and Repair Cost · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Blown Head Gasket Symptoms: The Signs and How to Test · Car Jerks When Accelerating: Causes, Costs and Fixes · and 33 other articles

The seal between the cylinder head and the block, holding back combustion pressure, coolant and oil at the same joint. It's a cheap part with an expensive job: replacing it means dismantling the top of the engine, so the labor is most of the bill and the gasket itself is almost none of it.

Where this comes up: Head Gasket Replacement Cost and Repair: Is It Worth Fixing? · 100k Mile Maintenance Cost: Every Quote Stops at 75,000 · 22 High-Mileage Cars to Avoid: Ranked Worst First · Blown Head Gasket Symptoms: The Signs and How to Test · and 19 other articles

An engine in which the valves and pistons occupy the same space at different moments. If the timing drive fails they meet, and a belt job becomes an engine rebuild. Most modern engines are of this type.

Where this comes up: Timing Belt Replacement Cost: Parts, Labor and Water Pump · Timing Belt Symptoms: What 4,315 Complaints Actually Show · Wet Belt Engines: What They Are and How They Fail · Wet Belt vs Timing Chain vs Dry Belt: Which You Have

A cylinder that doesn't burn its fuel properly, or at all. The unburned fuel leaves through the exhaust and burns inside the catalytic converter instead, which is why a flashing check engine light is treated as more urgent than a steady one.

Where this comes up: Flashing Check Engine Light: $256 Turning Into $1,511 · 11 Services Shops Push That Your Car Doesn't Need · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Blown Head Gasket Symptoms: The Signs and How to Test · and 35 other articles

The mesh strainer guarding the oil pump inlet. It's what catches belt debris in a wet belt engine, and once it blinds over the oil pressure falls, which is how a worn belt becomes a bearing failure.

Where this comes up: Wet Belt Engines: What They Are and How They Fail · Ford 1.0 EcoBoost Wet Belt: Is Your Car in the Recall? · Oil Light On? It Isn't Your Oil Change Reminder · Timing Belt Replacement Cost: Parts, Labor and Water Pump · and 3 other articles

A sensor in the exhaust that reports how much oxygen is left after combustion, which is how the engine corrects its own fuel mixture. Cars carry at least two: one before the catalytic converter doing that job, and one after it, whose only purpose is to check the converter is still working. Confusing the two is how people end up replacing a converter that was fine.

Where this comes up: P0420 Code: Bank 1 Meaning, Causes and Repair Cost · Bad or Clogged Catalytic Converter Symptoms: How to Tell · Blown Head Gasket Symptoms: The Signs and How to Test · Car Jerks When Accelerating: Causes, Costs and Fixes · and 15 other articles

A toothed drive belt that runs submerged in engine oil rather than in dry air. It's quieter and more compact than a chain, and it sheds material into the oil for its entire life, which is where the failures come from.

Where this comes up: Wet Belt Engines: What They Are and How They Fail · 0W-20 Substitute: 5W-20 and Almost Nothing Else · 100k Mile Maintenance Cost: Every Quote Stops at 75,000 · Can You Mix Synthetic and Conventional Oil? Two Questions · and 11 other articles

A small radiator inside the dashboard that heats the cabin. The part is cheap and the job isn't: on many cars the dash has to come out to reach it, which is why a heater core leak costs more to fix than a radiator that holds several times as much coolant.

Where this comes up: Coolant Leak Repair Cost: How Much to Fix Each Leak · Blend Door Actuator: Symptoms, Location, Reset and Cost · Can You Mix Coolant Colors? The Color Isn't the Answer · Car Fluid Leak Repair Cost: Which Ones Can Wait · and 6 other articles

Hybrid Organic Acid Technology: organic inhibitors with silicates or phosphates added, common in European and some Asian vehicles. Mixing it with a straight OAT product is where most coolant trouble starts.

Where this comes up: Can You Mix Coolant Colors? The Color Isn't the Answer · What Coolant Does My Car Need? Find the Specification

Inorganic Acid Technology: the older green coolant with silicate inhibitors that deplete in about two years. Still correct for older vehicles, and wrong for almost anything modern.

Where this comes up: Can You Mix Coolant Colors? The Color Isn't the Answer

Organic Acid Technology: long-life coolant using organic inhibitors, typically good for five years or more. Color tells you nothing reliable about which family is in the bottle, because two makers use the same dye for different chemistry.

Where this comes up: Can You Mix Coolant Colors? The Color Isn't the Answer · Coolant Flush Cost: Is Your Quote Inflated? · What Coolant Does My Car Need? Find the Specification

A temperature-operated valve that blocks flow to the radiator until the engine reaches operating temperature. Stuck shut, the engine overheats; stuck open, it never warms up properly and fuel economy drops, which is the failure people ignore because nothing appears to be wrong.

Where this comes up: Coolant Flush Cost: Is Your Quote Inflated? · Blown Head Gasket Symptoms: The Signs and How to Test · Can You Mix Coolant Colors? The Color Isn't the Answer · Check Engine Light Codes: 26 OBD-II Codes, 6 Families · and 7 other articles

The pump that circulates coolant, driven by the timing belt or chain on most engines. That last part is why it is usually replaced with the belt rather than on its own: the labor to reach it has already been paid for, and doing it later means paying that labor twice.

Where this comes up: Coolant Leak Repair Cost: How Much to Fix Each Leak · 100k Mile Maintenance Cost: Every Quote Stops at 75,000 · Antifreeze vs Coolant: Are They the Same Thing? · Blown Head Gasket Symptoms: The Signs and How to Test · and 10 other articles

The stacks of friction plates inside an automatic that fluid pressure clamps together to hold each gear. Worn clutch packs are what the word rebuild refers to, and grit or metal in the fluid is the sign they have gone.

Where this comes up: Transmission Slipping: Signs, Causes and the Fix · 22 High-Mileage Cars to Avoid: Ranked Worst First · Bad Transmission Symptoms: Is Your Transmission Going Out? · Car Jerks When Accelerating: Causes, Costs and Fixes · and 1 other article

A drain and fill replaces the fluid the pan holds, a third to a half of the total. A flush pumps fluid through the whole circuit and exchanges nearly all of it. They're different jobs at different prices and are frequently sold as if they were the same one.

Where this comes up: Is a Transmission Flush Necessary or a Fluid Change? · 11 Services Shops Push That Your Car Doesn't Need · Bad Transmission Symptoms: Is Your Transmission Going Out? · CVT Transmission Problems: It Depends Whose CVT · and 4 other articles

The electrically worked valve the transmission computer uses to decide which clutches engage and when. When one fails the timing goes wrong, so shifts come late or hard, and the fault usually sets a code that names it.

Where this comes up: Bad Transmission Symptoms: Is Your Transmission Going Out? · Transmission Slipping: Signs, Causes and the Fix

The fluid coupling between engine and automatic transmission, doing the job a clutch does on a manual. It multiplies torque from a standstill and then locks up at cruising speed to stop wasting fuel. A shudder at steady speed is usually that lock-up clutch rather than the transmission itself.

Where this comes up: Bad Transmission Symptoms: Is Your Transmission Going Out? · Car Jerks When Accelerating: Causes, Costs and Fixes · Car Shuts Off While Idling or Driving? Causes and What to Do · Check Transmission Fluid Without a Dipstick: 3 Jobs · and 5 other articles

The block of passages and valves that routes fluid to each clutch, with the shift solenoids working through it. A valve body fault can make a transmission slip with no mechanical wear at all, and usually sets a code.

Where this comes up: Bad Transmission Symptoms: Is Your Transmission Going Out? · Car Jerks When Accelerating: Causes, Costs and Fixes · P0700 Code: The One That Points at Another Code · P0720 Code: Output Speed Sensor Causes and Free Recalls · and 1 other article

The published number of hours a repair is deemed to take, from an industry labor guide. Charging book time is normal and not a trick, but it's the line worth asking about when two quotes for the same job differ.

Where this comes up: How to Read a Repair Estimate Without Getting Overcharged · ABS Sensor: Wheel Speed Sensor Symptoms, Location, Cost · Bad Alternator or Battery? How to Tell Before You Buy · Bad Wheel Bearing Symptoms and How Long You Can Drive · and 17 other articles

A manufacturer paying for a repair it's not contractually obliged to cover, usually to limit reputational damage from a known fault. Terms are discretionary and usually capped by age and mileage, so what you get depends partly on asking.

Where this comes up: PureTech 1.2 Wet Belt: What Stellantis Covers · Catalytic Converter Replacement Cost: When It's $0 · Ford 1.0 EcoBoost Wet Belt: Is Your Car in the Recall?

Nearly identical words for two different prices. An OEM part comes in the carmaker's box; an OE part is the very same component from the supplier who made it for them, in that supplier's own box, and usually costs less. Most people read the two as synonyms and pay the difference.

Where this comes up: Can You Mix Coolant Colors? The Color Isn't the Answer · How to Read an Oil Bottle: API, ILSAC, OEM Approvals · Resurface or Replace Brake Rotors? One Measurement

A written instruction a manufacturer sends its own dealers, naming a fault, the repair and who pays for it. It's not a recall, and nobody writes to you about one, which is why an owner can be covered and never find out. Asking whether one covers your car costs nothing and occasionally costs the shop the whole bill.

Where this comes up: P0301 Code: A Plug, a Coil, or Coolant in the Cylinder · 0W-20 vs 5W-30: What Happens If You Use the Wrong One · ABS Light On: What It Means and Is It Safe to Drive? · Bad Transmission Symptoms: Is Your Transmission Going Out? · and 39 other articles