What Is a Fault Code (DTC)?
A diagnostic trouble code (DTC) is simply the way your car tells you something is wrong. When a sensor reading goes outside normal range, the car’s computer stores a code like P0420 or P0301. Reading that code with a diagnostic tool is the fastest way to know where to look.
Codes are clues, not verdicts – two different faults can produce the same code, and a code doesn’t tell you which part is worn. The tables below give plain-English meanings and the most common causes to check. If you’re new to diagnostics, start with our explainer on what OBD is.
How to Read a Fault Code
Every code has one letter and four numbers. The letter tells you which system:
- P – Powertrain (engine, gearbox, emissions)
- B – Body (lights, windows, airbag, interior)
- C – Chassis (brakes, steering, suspension)
- U – Network (communication between modules)
The first number is 0 for a standardised (generic) code that works across all makes, or 1 for a manufacturer-specific code. The last three numbers narrow it down further (P0301 = cylinder 1 misfire).
Engine & Fuel Codes
| Code | What it means | Most common causes |
|---|---|---|
| P0300 | Random / multiple cylinder misfire | Spark plugs, ignition coils, fuel supply, vacuum leak |
| P0301–P0308 | Misfire on a specific cylinder | That cylinder’s plug, coil, injector or compression |
| P0171 / P0174 | Mixture too lean (bank 1 / bank 2) | Vacuum or air leak, MAF sensor, fuel pressure, O2 sensor |
| P0172 / P0175 | Mixture too rich (bank 1 / bank 2) | Dirty or faulty MAF, fuel pressure, leaking injector |
| P0106 | MAP sensor range / performance | Faulty manifold pressure sensor, intake leak, wiring |
| P0113 / P0112 | Intake air temp sensor signal high / low | Sensor fault, wiring or connector |
| P0117 / P0118 | Engine coolant temp sensor signal high / low | Sensor fault, wiring, low coolant |
| P0128 | Engine not reaching thermostat temperature | Thermostat stuck open |
| P0455 / P0442 | Large / small fuel vapour leak | Fuel cap loose, purge valve, hoses |
Emission & Exhaust Codes
| Code | What it means | Most common causes |
|---|---|---|
| P0420 / P0430 | Catalytic converter efficiency below threshold (bank 1 / bank 2) | Failing catalyst, oxygen sensors, exhaust leak |
| P0401 | EGR (exhaust gas recirculation) flow insufficient | Sooted-up EGR valve, blocked passages |
| P0402 | EGR flow excessive | EGR valve stuck open |
Diesel, DPF & AdBlue Codes
| Code | What it means | Most common causes |
|---|---|---|
| P20BD | AdBlue / NOx catalyst heating circuit | Heater element, wiring, fuse |
| P2BA9 or NOx codes | NOx catalyst or dosing system efficiency | AdBlue quality, blocked injector, dosing pump, DPF soot |
| P2453 / P2463 | DPF pressure sensor range / soot mass too high | DPF blocked, sensor, failed regeneration |
| P2031 | Exhaust gas temperature sensor (bank 2) | Sensor, wiring |
Diesel codes are often linked. If an AdBlue or NOx code appears, poor fluid or a blocked system is a frequent underlying cause – and it’s worth being cautious about treatments: see our AdBlue system recovery fluid and the Mercedes AdBlue fault fix guide before spending on parts.
Transmission Codes
| Code | What it means | Most common causes |
|---|---|---|
| P0700 | Transmission control module requests the engine light | TCM (gearbox computer) fault – read the sub-code |
| P0730 | Incorrect gear ratio | Low fluid, worn clutch/bands, solenoid |
| P0741 | Torque converter clutch stuck off | TCC solenoid, transmission internal |
Brakes & Speed Codes
| Code | What it means | Most common causes |
|---|---|---|
| P0500 | Vehicle speed sensor | Sensor, wiring, tone ring |
| C0035 | Left front wheel speed sensor circuit | Sensor, harness, damaged ring |
Network & Module Communication Codes
| Code | What it means | Most common causes |
|---|---|---|
| U0100 | Lost communication with the engine control module | Wiring, module power/earth, CAN bus fault |
| U0101 | Lost communication with the gearbox control module | Wiring, module power/earth |
| U0121 | Lost communication with the ABS module | Wiring, ABS module supply |
Manufacturer-Specific Codes
Codes starting with a second digit 1 (e.g. P1606) are defined by the manufacturer, not the industry standard. The same text can mean different things on different makes – a dealer-level tool or a brand guide is the reliable way to read them. For example, Mercedes fault codes have their own meanings; we explain the common ones in our Mercedes fault code guide.
One worth knowing: P0606 (engine control processor failure) is a module-level fault that usually needs professional dealer-level diagnosis rather than a simple part swap.
What to Do With Your Code
- Don’t panic at the code name. “Catalyst below threshold” doesn’t always mean a new cat – check the simpler causes first (as above).
- Clear the code and see if it returns. Intermittent faults sometimes never come back. If the light relights within a few drives, the fault is real.
- Fix the cause, not the symptom. Clearing a code without fixing the problem just makes the light return.
- Unsure? Get live data. A scanner that shows sensor readings tells you more than the code alone.
Which Tool Reads These Codes?
- Any OBD2 scanner reads generic codes (P, B, C, U). A simple ELM327 USB reader is fine for home owners.
- A full scanner like the Autel MaxiCOM MK808BT adds live data, reset and advanced service functions for garages.
- Trucks and plant need specialist HGV diagnostic tools.
- Choosing between a code reader and a dealer-level system? See our car diagnostic tool buyer’s guide.
Haven’t found your code, or need help deciding what to check next? Message us with the code and your vehicle and we’ll point you in the right direction.
