DTC Fault Codes List – Most Common Car Fault Codes Explained 2026

A customer arrives with the check engine light on. You plug in the scanner and get P0420. Half your team reaches for the catalytic converter. The other half checks the oxygen sensor first. The team that checks the O2 sensor — and the exhaust system for leaks — will find the actual fault in most cases. The team that orders the cat first will be refunding labour and parts when the light returns. In 2026, knowing not just what a DTC code means, but what causes it and how to verify the diagnosis, is what separates consistent workshop revenue from consistent comebacks. This DTC fault codes list gives you both.

How to Use This DTC Fault Codes List

This list covers the 50 most frequently encountered diagnostic trouble codes across UK and European workshops in 2026, based on aggregate workshop data. For each code you'll find:

• The code and generic definition
• The most statistically common causes
• The first diagnostic step before touching parts

For full manufacturer-specific definitions and guided diagnostic procedures, use AllData repair data or Mitchell1 ProDemand through AutoFixData.

Powertrain — Emissions and Engine Management Codes

Code	Definition	Most Common Causes	First Test
P0011	Camshaft Position A — Over-Advanced (Bank 1)	Cam phaser wear, oil viscosity, VVT solenoid fault	Check oil level and condition; oil pressure test
P0016	Camshaft/Crankshaft Position Correlation (Bank 1)	Timing chain stretch, cam phaser failure, correlation sensor	Check timing chain slack; crank/cam sensor air gap
P0101	MAF Sensor Range/Performance	Dirty or failed MAF, air leak after MAF, wiring fault	Check for air leaks; clean MAF; observe live MAF g/s values
P0128	Coolant Temperature Below Thermostat Regulating Temperature	Failed thermostat (most common), coolant temp sensor	Observe coolant temp with scan tool on cold start — should reach 85–90°C within 10 minutes
P0171	System Too Lean (Bank 1)	Vacuum leak, dirty/failed MAF, low fuel pressure, injector issue	Check short-term and long-term fuel trims; check for vacuum leaks (smoke test)
P0172	System Too Rich (Bank 1)	Leaking injector, high fuel pressure, failed MAF (low reading)	Check fuel pressure; observe MAF values vs expected
P0299	Turbocharger/Supercharger A Underboost Condition	Boost leak, failed VNT/turbo actuator, wastegate stuck open	Smoke test for boost leaks; observe boost pressure live data
P0300	Random/Multiple Cylinder Misfire Detected	Coil packs, spark plugs, injectors, low compression, VVT	Check individual cylinder misfire counters to identify cylinder(s)
P0301–P0308	Cylinder 1–8 Specific Misfire	Coil, spark plug, injector, compression loss on specific cylinder	Swap coil to another cylinder — if misfire moves, coil fault confirmed
P0400	Exhaust Gas Recirculation Flow Malfunction	Blocked EGR valve or passages, failed EGR solenoid	Command EGR valve open and observe idle quality; check for DTC P040x companion codes

Catalyst and Oxygen Sensor Codes

Code	Definition	Most Common Causes	First Test
P0420	Catalyst System Efficiency Below Threshold (Bank 1)	Downstream O2 sensor, exhaust leak, fuel system issue, failed catalyst	Check for exhaust leaks; monitor upstream/downstream O2 waveforms on scope
P0430	Catalyst System Efficiency Below Threshold (Bank 2)	Same as P0420 on Bank 2 (V-engines)	Same as P0420 — identify Bank 2 first
P0130	O2 Sensor Circuit Malfunction (Bank 1, Sensor 1)	Failed O2 sensor, wiring open/short, exhaust leak near sensor	Observe O2 waveform on scope — should switch 0.1V–0.9V at frequency >2Hz
P0136	O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)	Failed downstream O2 sensor, wiring fault	Check sensor heater circuit resistance (usually 5–20Ω); observe live voltage

Fuel System Codes

Code	Definition	Most Common Causes	First Test
P0087	Fuel Rail/System Pressure Too Low	Failing fuel pump, clogged fuel filter, FPR fault	Measure fuel rail pressure at idle and under load
P0191	Fuel Rail Pressure Sensor Performance/Range	Fuel rail pressure sensor, fuel pump, wiring	Check fuel rail pressure directly vs sensor reading
P019C	Fuel Rail Pressure Too High (Port Injection)	FPR fault, HPFP (direct injection), blocked return line	Direct fuel pressure measurement at rail

EVAP and Emissions Codes

Code	Definition	Most Common Causes	First Test
P0440	Evaporative Emission Control System Malfunction	Missing/loose fuel cap, EVAP purge solenoid, charcoal canister	Check fuel cap first — retighten and clear code
P0442	EVAP System Small Leak Detected	Fuel cap seal, EVAP hose, purge valve	Smoke test the EVAP system
P0455	EVAP System Large Leak Detected	Fuel cap off/missing, major EVAP line disconnect	Check fuel cap immediately; remove and reinstall

Transmission and Gearbox Codes

Code	Definition	Most Common Causes	First Test
P0700	Transmission Control System Malfunction	Indicates a TCM-specific fault — always pull companion TCM codes	All-system scan for TCM-specific codes; do not diagnose P0700 alone
P0730	Incorrect Gear Ratio	Solenoid fault, mechanical clutch pack wear, wrong fluid, TCM	Check transmission fluid level and condition first
P0741	Torque Converter Clutch Solenoid Performance	TCC solenoid, TCC circuit, mechanical TCC fault	Command TCC with scan tool; observe RPM drop at lock-up

Network and Communication Codes (U-Codes)

Code	Definition	Most Common Causes	First Test
U0001	High Speed CAN Bus Communication Fault	Wiring fault, failing module, battery voltage issue	Battery load test first; measure CAN resistance at OBD port (normal: 55–65Ω)
U0100	Lost Communication with ECM/PCM	ECM power supply, CAN bus fault, module fault — rarely the ECM itself	Check ECM fuse and power supply; then CAN bus resistance
U0121	Lost Communication with ABS Control Module	ABS module power/ground, CAN bus fault, ABS module failure	Check ABS module power and ground circuits; CAN bus
U0140	Lost Communication with Body Control Module	BCM power/ground, ignition switch circuit, CAN bus	Check BCM fuse, power supply, and ignition input

Body Codes (B-Codes)

Code	Definition	Most Common Causes	First Test
B0001	Driver Airbag Circuit — Resistance High	SRS clock spring, wiring open, airbag module	Check SRS wiring resistance; clock spring resistance
B1001	Manufacturer-specific body code	Varies — requires OEM specific definition from AllData or AutoData	Look up manufacturer specific definition first

Frequently Asked Questions

How do I find the definition of a manufacturer-specific DTC?

Use a professional data platform with OEM-specific code libraries. ALLDATA, AutoData, and Mitchell1 ProDemand all include manufacturer-specific definitions for codes beginning with P1, P2, B1, B2, C1, C2, U1, and U2. A generic OBD2 reader will often display "Manufacturer Controlled" without a description.

Is it safe to clear DTCs without fixing the fault?

Clearing codes without repair is safe on some non-critical systems but inadvisable. For airbag, ABS, or stability systems, never clear codes until the root fault is identified and repaired. Clearing codes also deletes freeze frame data — the only record of conditions when the fault occurred.

Why does the same code keep coming back after repair?

The code points to a system or circuit, not a specific component. Replacing the component that most commonly causes the code is correct — but only if that component was actually faulty. Always confirm with measurement before replacing. If the code returns after replacement, investigate the broader circuit (wiring, power supply, ground, related sensors).

What does it mean when multiple unrelated codes appear at once?

Multiple unrelated codes typically share a root cause — commonly a battery voltage problem, a CAN bus communication failure, or a shared ground point fault. Battery load test and CAN bus resistance check should be the first tests before addressing individual codes.

How do I access the full DTC diagnostic procedure for a specific code?

Professional data platforms like AllData and Mitchell1 ProDemand provide guided step-by-step diagnostic procedures for every DTC, including test specifications, wiring diagrams, and TSB cross-references.

Conclusion

A DTC fault code is an instruction to begin an investigation — not a permission slip to order a part. Every code on this list has multiple possible causes. The workshops that consistently diagnose correctly are those who treat the code as a starting address and use professional data tools to trace the physical evidence. In 2026, that combination of scanner data plus OEM repair data plus systematic testing is the baseline of professional workshop diagnosis.

External References: ASE automotive certification | official US vehicle recall database | AllData official website | ProDemand by Mitchell 1