Symptoms & Diagnosis
Something has changed and you want to know how worried to be. These guides start from what you can hear, feel or smell, then work through the causes in the order a sensible shop would check them: cheapest first, not most profitable first.
IndexEvery symptom we cover, and the article that answers it
Nothing happens when I turn the key
- Car Won't Start? Try This Before You Call a Tow
- Bad Alternator or Battery? How to Tell Before You Buy
It clicks and won't start
- Car Won't Start? Try This Before You Call a Tow
- Bad Alternator or Battery? How to Tell Before You Buy
It turns over slowly
- Car Won't Start? Try This Before You Call a Tow
- Bad Alternator or Battery? How to Tell Before You Buy
It turns over but won't fire
- Car Won't Start? Try This Before You Call a Tow
- Timing Belt Symptoms: What 4,315 Complaints Actually Show
A squeal when I brake
A grinding when I brake
- Warped Rotors: One Word, Three Faults, Three Bills
- Brake Pad Replacement Cost: With or Without Rotors?
A knock or clunk when I brake
A knocking or tapping from the engine
A whine that rises with speed
- Whining Noise When Accelerating: Transmission or Belt?
- CVT Transmission Problems: It Depends Whose CVT
A rattle for the first seconds after a cold start
- Rattling Noise When Accelerating: Causes and When to Stop
- Timing Belt Symptoms: What 4,315 Complaints Actually Show
The exhaust got louder
A loud roar the moment it starts
- Catalytic Converter Theft: What Actually Stops It
- Catalytic Converter Replacement Cost: When It's $0
Something smells like burning
- Bad Transmission Symptoms: Seven Causes Ranked by Cost
- Car Smells Like Burning: What Each One Costs, When to Stop
It smells like hot or burnt oil
- Car Smells Like Burning: What Each One Costs, When to Stop
- Oil Pan Gasket Replacement Cost: Parts, Labor and Quotes
- Valve Cover Gasket Replacement Cost: $300 to $500
A sweet, syrupy smell
A rotten-egg smell
I can smell gasoline
- Why Does My Car Smell Like Gas? Causes, Costs and Fixes
- P0455 Code: Large EVAP Leak Cost, Starting at the Cap
- What Is Oil Dilution? Fuel in Your Oil, and the Limits
- Honda CR-V 2017-2022 Oil Dilution: The 1.5T Problem
A hot plastic or electrical smell
White smoke from the exhaust
- Blown Head Gasket Symptoms: The Signs and How to Test
- Head Gasket Replacement Cost and Repair: Is It Worth Fixing?
- How Long Can You Drive With White Smoke From Exhaust?
Blue smoke from the exhaust
Smoke or steam from under the hood
The check engine light is on
- P0011 Code: Check the Oil Before You Buy a Solenoid
- P0302 Code: Cylinder 2 Misfire Causes, Fixes and Cost
- P0299 Code: Turbo Underboost Meaning, Causes and Fix
- P0303 Code: Is Your Cylinder 3 Misfire a Cheap Fix?
- P0304 Code: Cylinder 4 Misfire Detected and How to Fix It
- P0720 Code: Output Speed Sensor Causes and Free Recalls
- P0440 Code: The EVAP Code That Names Nothing
- P0174 Code: The Bank 2 Code That Isn't About Bank 2
- P0172 Code Meaning: Too Much Fuel and How to Fix It
- P0301 Code: A Plug, a Coil, or Coolant on a Ford
- P0700 Code: The One That Points at Another Code
- P0441 Code: Incorrect Purge Flow and What the Fix Costs
- P0442 Code: Why a New Gas Cap Often Changes Nothing
- P0128 Code: The Engine Never Got Warm Enough to Test Itself
- P0446 Code: A Valve Ignored an Order, Not a Leak
- P0171 Code: The Correction Ran Out, Not the Fuel
- Do Catalytic Converter Cleaners Work? Two Cases of Six
- P0443 Code Meaning: Purge Valve Circuit, Causes and Cost
- Check Engine Light Meaning: The Test Your Car Failed Twice
- P0455 Code: Large EVAP Leak Cost, Starting at the Cap
- Check Engine Light Codes: 26 OBD-II Codes, 6 Families
- P0456 Code: Will It Clear Itself, and What It Costs
- Bad Catalytic Converter Symptoms: Two Failures, Two Answers
- P0430 Code: Bank 2 Location and What Actually Fails
- P0420 Code: The Converter Is Usually the Victim
The check engine light is flashing
- P0302 Code: Cylinder 2 Misfire Causes, Fixes and Cost
- P0303 Code: Is Your Cylinder 3 Misfire a Cheap Fix?
- P0304 Code: Cylinder 4 Misfire Detected and How to Fix It
- P0301 Code: A Plug, a Coil, or Coolant on a Ford
- P0300 Code: Random Misfire, and the Two Tiers Behind It
- Flashing Check Engine Light: $256 Turning Into $1,511
The oil pressure light came on
- Oil Light On? It Isn't Your Oil Change Reminder
- Do High-Mileage Cars Burn Oil? Is a Quart per 1,000 Normal
An oil change or service reminder
The temperature light came on
The battery or charging light
- Dashboard Warning Lights: Three Mean Stop, Five Can Wait
- How Long Does an Alternator Last? 80,000 to 150,000 Miles
- Bad Alternator or Battery? How to Tell Before You Buy
The ABS light
- How to Test a Wheel Speed Sensor: Passive and Active
- ABS Sensor: Wheel Speed Sensor Symptoms, Location, Cost
- ABS Light On: What It Means and Is It Safe to Drive?
- Dashboard Warning Lights: Three Mean Stop, Five Can Wait
The traction control light
The brake warning light
- Service Brake Assist: What It Means, Causes and Cost
- Brake Warning Light: What It Means and Is It Safe to Drive?
- Dashboard Warning Lights: Three Mean Stop, Five Can Wait
The tire pressure light
A light I can't identify
The temperature gauge is reading high
- Blown Head Gasket Symptoms: The Signs and How to Test
- Head Gasket Replacement Cost and Repair: Is It Worth Fixing?
- Coolant Leak: The Five Places It Comes From
The temperature gauge never reaches normal
It is using more fuel than it used to
A puddle under the front that smells sweet
- Coolant Stop Leak: When It Works and What It Wrecks
- Coolant Leak Repair Cost: $496 to $1,902, by Leak Point
- Coolant Leak: The Five Places It Comes From
- Car Fluid Leak Repair Cost: Which Ones Can Wait
An oily patch under the front
An oily patch under the middle of the car
- Transmission Fluid Color: Dark Is Normal and Milky Isn't
- Car Fluid Leak Repair Cost: Which Ones Can Wait
- Transmission Leak Repair Cost: Cheap Fix or Big Job?
- Oil Pan Gasket Replacement Cost: Parts, Labor and Quotes
A thin, slippery wet patch behind a wheel
- Car Fluid Leak Repair Cost: Which Ones Can Wait
- Brake Fluid Leak Repair Cost: Caliper or Master Cylinder
A wet patch that is gone by morning
A level that drops with nothing on the ground
- Blown Head Gasket Symptoms: The Signs and How to Test
- Does Coolant Evaporate? The Water Goes, the Antifreeze Stays
- Coolant Stop Leak: When It Works and What It Wrecks
- Coolant Leak Repair Cost: $496 to $1,902, by Leak Point
- Head Gasket Replacement Cost and Repair: Is It Worth Fixing?
- Coolant Leak: The Five Places It Comes From
The steering wheel shakes when braking
- 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 brake pedal feels soft or sinks
The brake pedal pulses under my foot
- Warped Rotors: One Word, Three Faults, Three Bills
- Resurface or Replace Brake Rotors? One Measurement
It slips or hangs between gears
- Low Transmission Fluid Symptoms and When to Stop Driving
- P0700 Code: The One That Points at Another Code
- Transmission Slipping: What It Is, and What Waiting Costs
- CVT Transmission Problems: It Depends Whose CVT
- Bad Transmission Symptoms: Seven Causes Ranked by Cost
It jerks or shudders changing gear
- P0720 Code: Output Speed Sensor Causes and Free Recalls
- Whining Noise When Accelerating: Transmission or Belt?
- Low Transmission Fluid Symptoms and When to Stop Driving
- Car Jerks When Accelerating: Causes, Costs and Fixes
- P0700 Code: The One That Points at Another Code
- Transmission Slipping: What It Is, and What Waiting Costs
- CVT Transmission Problems: It Depends Whose CVT
- Bad Transmission Symptoms: Seven Causes Ranked by Cost
A vibration that comes with speed
- Car Jerks When Accelerating: Causes, Costs and Fixes
- Steering Wheel Shakes When Braking: 5 Causes by Cost
It idles roughly or shakes at a stop
- P0011 Code: Check the Oil Before You Buy a Solenoid
- P0302 Code: Cylinder 2 Misfire Causes, Fixes and Cost
- P0303 Code: Is Your Cylinder 3 Misfire a Cheap Fix?
- P0304 Code: Cylinder 4 Misfire Detected and How to Fix It
- P0174 Code: The Bank 2 Code That Isn't About Bank 2
- P0301 Code: A Plug, a Coil, or Coolant on a Ford
- P0171 Code: The Correction Ran Out, Not the Fuel
- P0300 Code: Random Misfire, and the Two Tiers Behind It
- Flashing Check Engine Light: $256 Turning Into $1,511
It hesitates or stumbles when I accelerate
- Car Jerks When Accelerating: Causes, Costs and Fixes
- P0171 Code: The Correction Ran Out, Not the Fuel
- P0300 Code: Random Misfire, and the Two Tiers Behind It
It has lost power
- P0299 Code: Turbo Underboost Meaning, Causes and Fix
- Whining Noise When Accelerating: Transmission or Belt?
- Engine Power Reduced: Meaning, Causes and How to Fix
- Car Jerks When Accelerating: Causes, Costs and Fixes
- Transmission Slipping: What It Is, and What Waiting Costs
- Bad Catalytic Converter Symptoms: Two Failures, Two Answers
- Timing Belt Symptoms: What 4,315 Complaints Actually Show
It stalls
It went into limp mode
- P0299 Code: Turbo Underboost Meaning, Causes and Fix
- P0720 Code: Output Speed Sensor Causes and Free Recalls
- Engine Power Reduced: Meaning, Causes and How to Fix
- Bad Transmission Symptoms: Seven Causes Ranked by Cost
It failed an emissions or smog test
- P0440 Code: The EVAP Code That Names Nothing
- P0174 Code: The Bank 2 Code That Isn't About Bank 2
- P0172 Code Meaning: Too Much Fuel and How to Fix It
- P0441 Code: Incorrect Purge Flow and What the Fix Costs
- P0442 Code: Why a New Gas Cap Often Changes Nothing
- Do Catalytic Converter Cleaners Work? Two Cases of Six
- P0455 Code: Large EVAP Leak Cost, Starting at the Cap
- Catalytic Converter Replacement Cost: When It's $0
- P0456 Code: Will It Clear Itself, and What It Costs
- Bad Catalytic Converter Symptoms: Two Failures, Two Answers
- P0430 Code: Bank 2 Location and What Actually Fails
- P0420 Code: The Converter Is Usually the Victim
The test said the monitors aren't ready
- P0011 Code: Check the Oil Before You Buy a Solenoid
- P0302 Code: Cylinder 2 Misfire Causes, Fixes and Cost
- Blend Door Actuator: Symptoms, Location, Reset and Cost
- How to Test a Wheel Speed Sensor: Passive and Active
- Service Brake Assist: What It Means, Causes and Cost
- Car Shuts Off While Idling or Driving? Causes and What to Do
- Blown Head Gasket Symptoms: The Signs and How to Test
- ABS Sensor: Wheel Speed Sensor Symptoms, Location, Cost
- Traction Control Light: Flashing vs Steady, and Is It Safe?
- Knocking Noise When Braking: 8 Causes and Is It Safe?
- ABS Light On: What It Means and Is It Safe to Drive?
- Brake Warning Light: What It Means and Is It Safe to Drive?
- P0299 Code: Turbo Underboost Meaning, Causes and Fix
- P0303 Code: Is Your Cylinder 3 Misfire a Cheap Fix?
- Rattling Noise When Accelerating: Causes and When to Stop
- P0304 Code: Cylinder 4 Misfire Detected and How to Fix It
- P0720 Code: Output Speed Sensor Causes and Free Recalls
- Whining Noise When Accelerating: Transmission or Belt?
- Low Transmission Fluid Symptoms and When to Stop Driving
- Car Won't Start? Try This Before You Call a Tow
- Engine Power Reduced: Meaning, Causes and How to Fix
- Car Jerks When Accelerating: Causes, Costs and Fixes
- Why Does My Car Smell Like Gas? Causes, Costs and Fixes
- P0440 Code: The EVAP Code That Names Nothing
- P0174 Code: The Bank 2 Code That Isn't About Bank 2
- P0172 Code Meaning: Too Much Fuel and How to Fix It
- P0301 Code: A Plug, a Coil, or Coolant on a Ford
- P0700 Code: The One That Points at Another Code
- P0441 Code: Incorrect Purge Flow and What the Fix Costs
- P0442 Code: Why a New Gas Cap Often Changes Nothing
- P0128 Code: The Engine Never Got Warm Enough to Test Itself
- P0446 Code: A Valve Ignored an Order, Not a Leak
- Oil Light On? It Isn't Your Oil Change Reminder
- Oil Change Light: What the Percent Means, How to Reset It
- P0171 Code: The Correction Ran Out, Not the Fuel
- P0300 Code: Random Misfire, and the Two Tiers Behind It
- Transmission Slipping: What It Is, and What Waiting Costs
- CVT Transmission Problems: It Depends Whose CVT
- P0443 Code Meaning: Purge Valve Circuit, Causes and Cost
- Dashboard Warning Lights: Three Mean Stop, Five Can Wait
- Bad Transmission Symptoms: Seven Causes Ranked by Cost
- Check Engine Light Meaning: The Test Your Car Failed Twice
- P0455 Code: Large EVAP Leak Cost, Starting at the Cap
- Check Engine Light Codes: 26 OBD-II Codes, 6 Families
- P0456 Code: Will It Clear Itself, and What It Costs
- Bad Catalytic Converter Symptoms: Two Failures, Two Answers
- P0430 Code: Bank 2 Location and What Actually Fails
- Bad Wheel Bearing Symptoms and How Long You Can Drive
- Bad Alternator or Battery? How to Tell Before You Buy
- Flashing Check Engine Light: $256 Turning Into $1,511
- Coolant Leak: The Five Places It Comes From
- P0420 Code: The Converter Is Usually the Victim
- Timing Belt Symptoms: What 4,315 Complaints Actually Show
- Warped Rotors: One Word, Three Faults, Three Bills
- Car Smells Like Burning: What Each One Costs, When to Stop
- How Long Can You Drive With White Smoke From Exhaust?
- Steering Wheel Shakes When Braking: 5 Causes by Cost
How these diagnostic guides are written
A noise, a smell or a warning light has several possible answers and they rarely cost the same. If it is a warning light, the dashboard symbol board starts from the shape you can see. These guides rank the causes by what each one costs to fix, whether that is a car making noise when braking, a burning smell after driving or white smoke from the exhaust, and give the check that separates the cheap answer from the expensive one.
Cheapest first, on purpose
Causes are ordered by repair cost, not by how common they are. That's the order a shop should work through them rather than the order that bills best.
We don't have frequency data, and a cheap cause listed first isn't a prediction that yours is cheap.
Stop now, or book it in
Every guide separates the symptoms that mean stop driving from the ones that mean make an appointment. Where the difference turns on one observation, that check comes before any opinion.
What we don't do
Nothing here diagnoses your car, and no article can tell you which of several causes yours is on. Where an inexpensive test settles it, such as a pressure test or a dipstick read a hundred miles apart, that's the step we point at.
Found a figure here that doesn't match what you were quoted, or think something on this page is wrong? Tell us. Corrections get read first and the page gets dated when it changes. If you want to know how these guides are put together before you trust one, that's written down too.