Check Engine Light Codes: 29 OBD-II Codes in 6 Families

A check engine light code is an OBD-II code (on-board diagnostics, generation two), or DTC, a diagnostic trouble code. It's stored by the self-test system federal rules require on modern US cars.

If it's the lamp you're unsure about rather than the code, start at the symbol board. What the lamp is reporting, and why it waited before saying anything, is in our guide to check engine light meaning.

Can you keep driving with a check engine light code?

Yes, if the light is steady, as our guide to a light on with the car running fine explains. Three exceptions move a check engine light code out of wait and see:

1. A flashing light

Blinking and flashing are the same thing: a misfire severe enough that the exhaust is being damaged right now, so it belongs in the stop group. Our guide to the flashing check engine light covers what it's counting down to.

2. A code with a symptom

Overheating, a loss of power, a smell or a noise takes it out of the waiting group. That holds whatever the code says.

Our free symptom finder starts from the sound, the smell or the light. The other lamps worth knowing are in our guide to dashboard warning lights.

3. An inspection coming up

A lit lamp fails the test in states that run one, and California also fails a permanent code, which a scan tool can't erase. A car whose monitors, the self-tests for each system, haven't finished running since the last clear can fail too.

What do check engine light codes mean?

Each code names a test that failed and where it sits in the car. Codes cluster by system, and the six families in the code list below cover most of the ones you'll meet.

When one code leads to another

A P0300 misfire ignored long enough can kill the converter and set a P0420 later. Our guide to the P0420 code works that chain through in full.

Some generic codes matter because their failure stops the car's other tests from running. P0128, the thermostat code, is the clearest case: its first failure condition is that coolant stayed below the thermostat's temperature, and an engine that cold can hold up the car's other tests.

How do you read a check engine light code?

Plug a reader into the port under the dash on the driver's side, then read the five characters one at a time. Each position narrows the search, and the third one does most of the work.

Every OBD-II code is built the same way, so reading one is the same job every time. Here's how to read check engine codes, using P0420 one character at a time, in the order the drawing rings them:

• The P names the area. P is powertrain, which is the engine and transmission and almost everything that turns the light on. B is body, C is chassis, U is the network the modules talk over.
• The 0 tells you the meaning is the same on every car. A 0 means the code is generic, identical on every car sold here. A 1 means the manufacturer defined it, and its meaning is theirs.
• The 4 names the system, the character worth memorizing. Digits 1 and 2 are fuel and air, 3 ignition and misfire, 4 emission controls, 5 speed and idle, 6 the computer's outputs, 7 and 8 the transmission.
• The 20 is the fault inside that system, the only part you need to look up. On P0420 it's catalyst efficiency low on bank 1, the side with cylinder 1. A V6, a V8 or a flat engine has a second side, bank 2.

So P0420 reads as powertrain, generic, emission controls, fault 20. Even without knowing what 20 is, you already know it's an emissions problem on a standard code any reader can pull, and not a transmission or a network fault.

For any other code, type it in with your car's year and model. Use your carmaker's meaning for a code whose second character is 1.

Is it PO420 or P0420?

It's a zero, never the letter O. On a scan tool display the two are drawn almost identically, and typing PO420 instead of P0420 is the easiest mistake in this subject to make.

It matters because a parts catalog or a service database returns nothing at all rather than correcting you. That reads as though the code doesn't exist and can send you back to the shop for a second reading you don't need.

If you got here that way, you read the code correctly and only mistyped it, and the same applies to every code on this page: PO171 is P0171, PO300 is P0300, PO301 is P0301, PO456 is P0456. The second character is a digit, zero and not the letter O, and some codes end in letters, like P04EE.

OBD-II is on-board diagnostics, generation two, and the II is a Roman numeral rather than two ones. That's where OBD11 comes from, while OBD2 means the same thing and is what most people write.

How much does it cost to diagnose a check engine light?

Free at a parts counter, $38–$40 for your own dongle, and a shop's own fee, which this page doesn't price, so ask for it first. A code read and a paid diagnosis are two different purchases, and the gap between them is the whole answer:

Treating those as one purchase is how people end up buying a part on the strength of a number. Finding out what a code means on your engine is the work, and what a fair estimate for that work looks like is set out in our guide to reading a repair estimate.

Can I clear the code myself?

Not before the repair. Clearing wipes the freeze frame, the snapshot of what the engine was doing when the fault was recorded and the most useful thing a technician gets for free.

It also resets the emissions monitors, and those can take days of ordinary driving to run again. A permanent code survives the clear too.

Does the code tell me which part to replace?

No, and that's the expensive misunderstanding. A code reports a measurement outside its limits, and P0420 is the clearest case of it.

P0420 means catalyst efficiency below threshold on bank 1. It's a measurement of the converter's output, taken by the sensor behind it, and it reports that the catalyst has stopped storing oxygen.

It doesn't say the converter is what broke. The converter can be the victim: a misfire cooking it, an exhaust leak upstream of it, closer to the engine, letting in air that fools the sensor, or coolant getting past a head gasket and poisoning it.

Replace the converter without finding what killed it and the code comes back, on a new converter. That's the difference between a code and a diagnosis, and it's why the free reading at the parts counter is the beginning of the job rather than the end of it.

P0299, turbo underboost, the code for a turbo making less boost than asked, works the same way. GM wrote a bulletin listing when a turbo that set it shouldn't be replaced, because good turbos kept coming back from repairs with nothing wrong.

P0720, the code for the sensor that reads the transmission's output speed, is another. The makers' own documents blame the connector, the wiring and the parts around the sensor more often than the sensor.

The part a free code read leaves out

How much does a check engine light cost to fix?

It depends on the code, from a loose gas cap to a converter. One ignition coil on a 2025 Chevrolet Silverado 1500 5.3L runs $88–$242 at an independent shop, parts and labor.

A federal-grade catalytic converter runs $402–$1,846, parts and labor, on the cars our converter cost page prices. The code list above shows what each code we have written up costs, part plus booked labor.

What should you do about the code you have?

Write the code down before anything clears it, then act on it in this order:

1. Read the characters, not just the number. The letter and the first two digits tell you the area, whether any reader can pull it, and which system reported the fault. That's most of what a shop is about to tell you.
2. Check the three exceptions above. A flashing light, a symptom you can feel, or an inspection coming up changes what you do next, whatever the code says.
3. Read our page for that code if we have one. The index above links every code we have written up. Our free car complaints lookup shows what owners of your exact car have reported.
4. Ask what test turned the code into a part. The code names a measurement that failed. Turning that into a component is a diagnosis, and a shop that skips it is guessing with your money. Before you pay, our free VIN decoder lists the recalls for your model year, and NHTSA's VIN search shows which are still open on yours.

Common questions

Asked most often

Are the codes the same on every car?

The generic ones are, and that's why a basic reader can read them on any car. A 1 in the second position means the manufacturer defined that code themselves, and those vary by maker.

What is DTC in a car?

A diagnostic trouble code, which is the same thing as a check engine light code. Shops and scan tools write DTC, owners write code, and both mean the five characters the computer stored when a test failed.

What is OBD?

On-board diagnostics, the system in the car that runs the tests and stores the codes. OBD-II is generation two, required by federal rules, and it's why a basic reader works on any car that has it.

Prices are US figures and vary by region, engine and shop rate. The prices beside individual codes say on the row what they cover. This article is general information, not advice about your specific vehicle. See the disclaimer.