What does my trouble code mean?
Type the code from your scan tool or parts-store printout. We break down every character and link the full answer when we have one.
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Codes we’ve written full answers for
- P0128If the temperature gauge stays near C, or drops on the highway, after you've driven long enough to warm up, the most common cause is a thermostat stuck open: coolant flows through the radiator all the time, so the engine never reaches its normal temperature. The other usual causes are a faulty coolant temperature sensor or gauge, low coolant, or a cooling fan that won't shut off; it isn't an emergency like overheating, but get it fixed soon, because a cold-running engine burns more fuel, heats the cabin poorly and wears faster.
- P0171P0171 means "System Too Lean (Bank 1)" — the engine computer is adding as much fuel as it's allowed to and the mixture is still running lean, meaning too much air or too little fuel. The most common causes are a vacuum leak (a cracked intake boot, hose or PCV line), a dirty or failing mass airflow (MAF) sensor, and low fuel pressure, so start there rather than replacing oxygen sensors.
- P0300P0300 means "Random/Multiple Cylinder Misfire Detected" — the engine computer sees misfires that aren't tied to one cylinder. The usual suspects are worn spark plugs or ignition coils, a vacuum leak, low fuel pressure or a bad mass airflow sensor, and it's best not to keep driving on it, because a misfire can damage the engine and the catalytic converter.
- P0420P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)" — the engine computer has decided the catalytic converter on bank 1 isn't cleaning the exhaust as well as it should. The converter itself is often worn out, but an exhaust leak, a bad downstream oxygen sensor, or an engine problem that damaged the converter can set the same code.
- P0442P0442 means the evaporative emissions (EVAP) system, which holds gasoline vapor from the tank, found a small leak. Rick's Free Auto Repair Advice calls a loose or faulty gas cap by far the most common cause, followed by cracked EVAP hoses and a purge or vent valve that doesn't seal fully.
- P0455P0455 means the evaporative emissions (EVAP) system, which traps gasoline vapor from the fuel tank, found a large leak, roughly a hole 0.04 inch across or bigger. The most common cause is a loose, missing or worn gas cap, so check that first; after that the usual suspects are a stuck vent or purge valve, a cracked EVAP hose or a damaged charcoal canister.
- P0456P0456 means the evaporative emissions (EVAP) system, which keeps gasoline vapor from escaping the fuel tank, found a very small leak during its self-test. A loose or worn gas cap is the most common cause, so check it first; after that the usual suspects are a cracked EVAP hose, a purge or vent valve that doesn't seal, or a damaged charcoal canister.
How to read any code
1st character: system
- P
- Powertrain
- B
- Body
- C
- Chassis
- U
- Network (communication between the car’s control modules)
2nd character: code type
- 0
- Generic SAE code: the same meaning on every make
- 1
- Manufacturer-specific: meaning depends on the automaker
- 2
- Generic SAE code: the same meaning on every make
- 3
- Either generic or manufacturer-specific, depending on the exact code
3rd character on P codes: sub-system
- 0
- Fuel and air metering, and auxiliary emission controls
- 1
- Fuel and air metering
- 2
- Fuel and air metering (injector circuit)
- 3
- Ignition system or misfire
- 4
- Auxiliary emission controls
- 5
- Vehicle speed, idle control and auxiliary inputs
- 6
- Computer and output circuits
- 7
- Transmission
- 8
- Transmission
- 9
- Transmission
- A
- Hybrid propulsion
The last two characters pick the exact fault. On misfire codes they’re the cylinder: P0303 is cylinder 3.
A code tells you which test failed, not which part to buy. Some automakers vary from the standard definitions, so confirm the exact wording for your car before replacing anything.
Sources:Maryland Department of the Environment: Diagnostic Trouble Code (DTC) decoding; Haynes: What are diagnostic trouble codes?