A trouble code is not a diagnosis. It is the car telling you which circuit or system failed a self-test — the actual broken part still has to be found. These guides explain what each code means, which symptoms go with it, the causes that turn out to be responsible most often, and how to confirm the fault before you spend money on parts.
The Four Code Families
Every OBD-II code starts with a letter that tells you which part of the car reported it, followed by four digits.
How to Read a Code
Take P0301 as an example.
| P | The system. Powertrain, Chassis, Body or Unetwork. |
|---|---|
| 0 | Who defined it. 0 is a generic code shared by every manufacturer; 1 is specific to your make. |
| 3 | The subsystem — here, ignition and misfire. |
| 01 | The specific fault. In this case, a misfire detected on cylinder 1. |
A basic OBD-II scanner plugs into a port under the dashboard on the driver’s side and costs less than a single hour of workshop labour. Every petrol car sold in the US since 1996 and every petrol car sold in the EU since 2001 (diesel since 2004) has one. Many parts shops will also read codes free of charge.
Popular Trouble Code Guides
All code guides ›-
P0351 Code: Ignition Coil A Primary/Secondary Circuit
P0351 means the engine computer saw a fault in ignition coil A’s circuit: usually a loose connector, chafed wire or failed $40-$150…
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P2135 Code: Throttle Position Sensor A/B Voltage Correlation
P2135 means the two throttle position sensor signals disagree, so the engine cuts power. Usual causes: worn sensor or bad connector, $0-$800…
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P0606 Code: Control Module Processor Fault (ECM/PCM)
P0606 means the engine computer failed its own processor self-test. Check battery voltage (12.6 V at rest) and grounds before replacing the…
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P0457 Code: EVAP Leak Detected (Fuel Cap Loose/Off)
P0457 means the EVAP leak test failed soon after refuelling, so the fuel cap is the prime suspect. Causes, fixes from $0,…
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P0730 Code: Incorrect Gear Ratio (Causes and Fixes)
P0730 means the transmission’s measured gear ratio doesn’t match the commanded gear. Causes, diagnosis and repair costs from $20 to $6,000.
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P0118 Code: Engine Coolant Temperature Sensor Circuit High
P0118 means the coolant temperature sensor circuit reads high voltage (often -40°F): usually an open wire or failed $20-$60 sensor, not a…
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P0012 Code: Intake Camshaft Timing Over-Retarded (Bank 1)
P0012 means the intake camshaft is lagging behind where the computer wants it. Low or dirty oil and a sticky VVT solenoid…
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P0480 Code: Cooling Fan 1 Control Circuit
P0480 means the computer can’t control cooling fan 1. A blown fuse or failed relay is common — but a dead fan…
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P0441 Code: EVAP System Incorrect Purge Flow
P0441 means fuel vapor isn’t flowing the way the computer commands during purge. A stuck purge valve or blocked line is the…
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P0341 Code: Camshaft Position Sensor Range/Performance (Bank 1)
P0341 means the camshaft sensor’s signal is present but implausible. It’s often the sensor itself, but a stretched timing chain or low…
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P0336 Code: Crankshaft Position Sensor Range/Performance
P0336 means the crankshaft sensor’s signal is present but doesn’t make sense. It can cause stalling and no-start, and sometimes points to…
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P0299 Code: Turbocharger Underboost Condition
P0299 means the turbo is delivering less boost than the engine computer asked for. A split hose or a stuck wastegate is…
Before You Replace Anything
Codes point at circuits, not culprits. Three habits save most of the money wasted on guess-and-replace repairs:
- Write down every code and the freeze-frame data before clearing anything. Freeze frame captures engine temperature, load and speed at the moment the fault triggered — that context is often what identifies the cause.
- Fix the lowest code first when several appear together. A single lean-running fault can set half a dozen downstream codes that clear themselves once the root cause is gone.
- Check the cheap things first. Loose fuel caps, split vacuum hoses, dirty connectors and corroded earth straps cause an enormous share of codes that get blamed on sensors.
Chock the wheels, use axle stands rather than a jack alone, let hot components cool, and disconnect the battery before touching wiring. Brakes, steering, fuel lines, airbags and the orange high-voltage cabling on hybrids and EVs can cause serious injury or death if handled incorrectly. If a job is beyond your tools or experience, stop and book a qualified mechanic — there is no shame in it and it is always cheaper than the alternative.