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 ›-
U0121 Code: Lost Communication With ABS Control Module
This completes the network-fault family alongside U0100 and U0101 — the same category of problem, this time affecting the module responsible for…
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P0463 Code: Fuel Level Sensor Circuit High Input
This code and a dead fuel gauge are usually two symptoms of the exact same failed part — the sending unit inside…
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P0456 Code: EVAP System Very Small Leak Detected
This completes a three-tier leak-size hierarchy — very small, small, and gross — and it’s the one most often caused by nothing…
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P0303 Code: Cylinder 3 Misfire Detected
With P0301 and P0304 already covered, P0303 completes the most commonly searched trio of single-cylinder misfire codes — the diagnostic approach is…
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P0201 Code: Injector Circuit Malfunction – Cylinder 1
An injector circuit fault stops fuel delivery to one cylinder entirely — a more absolute problem than the partial misfire causes covered…
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P0230 Code: Fuel Pump Primary Circuit Malfunction
This is an electrical circuit fault, not a description of the pump’s mechanical condition — a healthy pump with a bad relay…
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P0122 Code: Throttle Position Sensor Circuit Low Input
This sensor tells the ECU how far the throttle is actually open — a low reading can trigger a defensive reduced-power response…
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Car Stuck in Limp Mode: What It Means and What to Do
Limp mode is the car protecting itself, not breaking down — but driving through it without diagnosing the trigger just delays finding…
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P0500 Code: Vehicle Speed Sensor Circuit Malfunction
This one sensor affects the speedometer, cruise control, and automatic shift timing all at once — a fault here can look like…
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P0562 Code: System Voltage Low
This code names a symptom — low voltage — rather than a specific failed part, and the real question it raises is…
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P0135 Code: O2 Sensor Heater Circuit (Bank 1 Sensor 1)
The upstream twin of P0141 — same heater circuit fault, but on the sensor that actually controls fuelling, which changes what you…
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.