If your car has stored a fault code such as P0420, P0300, P0171, P0101, P0335, P0440, P0500, P0562, P0700 or P2195, this hub explains what the code means, symptoms to watch for, common causes, likely UK repair costs, whether you can still drive and whether the fault may affect an MOT.
Enter a fault code such as P0420, P0300, P0171, P0335, P0440 or P0700. If we already have a full guide for it, you can jump straight to the mechanic-style explanation.
This lookup currently supports the live fault-code pages already published on Motor Vehicle Expert. For other codes, use the main search bar above or browse the full list below.
An OBD fault code tells you which system the car has detected a problem with. It is a starting point, not a guaranteed parts list. A P0171 lean code may be caused by an air leak, fuel pressure issue or MAF problem. A P0335 crank sensor code may cause a crank-no-start, but wiring and live RPM data still need checking. A P0420 catalyst code may follow misfires, exhaust leaks or oxygen sensor faults.
If you're scanning a vehicle before buying it, our Used Car Buying Guides UK hub explains how to check MOT history, service records, dashboard warning lights, test drive behaviour, paperwork and common scams before committing to a purchase.
Write down stored, pending and permanent codes before clearing anything. Multiple codes together often tell a better story than one code on its own.
Misfire, smoke, limp mode, rough idle, warning lights, no-start and loss of power matter as much as the code itself.
Use live data, visual checks, smoke testing, voltage testing and pressure checks before replacing parts.
These are the main live code guides in your Motor Vehicle Expert fault-code cluster.
Catalyst system efficiency below threshold.
Random or multiple cylinder misfire.
System too lean bank 1.
Camshaft timing over advanced.
Mass air flow sensor range/performance.
Intake air temperature sensor high input.
Coolant temperature below thermostat regulating temperature.
Crankshaft position sensor circuit fault.
Transmission control system malfunction.
These codes often cause rough idle, hesitation, poor acceleration, poor economy, stalling or limp mode.
Airflow reading outside expected range. Check MAF, intake leaks and air filter.
IAT sensor or wiring fault causing high intake temperature signal.
Often thermostat stuck open, coolant sensor issue or slow warm-up.
Too much air or not enough fuel on bank 1.
Cylinder 1 injector circuit, wiring or injector fault.
Upstream oxygen or air-fuel sensor biased or stuck lean.
These codes are important because they can affect fuel mixture, catalyst health and MOT emissions.
Bank 1 sensor 1 heater circuit fault.
Bank 1 sensor 2 heater circuit fault after the catalyst.
Catalyst, oxygen sensor, exhaust leak or mixture-related issue.
Blocked EGR, carbon build-up or flow control issue.
EGR valve stuck open or excessive flow detected.
Understand common emissions-related MOT failures.
EVAP faults often trigger the engine light even when the car drives normally. Fuel smell or visible leakage should always be checked quickly.
General EVAP system fault or vapour control problem.
Small vapour leak, fuel cap seal or EVAP hose issue.
Vent valve, vent hose, canister or vent control circuit fault.
Large EVAP leak, missing cap, disconnected pipe or canister issue.
Useful if there is a fuel smell with EVAP codes.
Fuel vapour smell inside the car needs careful checking.
Turbo and EGR faults can cause limp mode, smoke, poor acceleration, poor fuel economy and emissions problems.
Boost pressure too high. Check actuator, boost control and turbo vanes.
Boost pressure too low. Check pipes, intercooler, actuator and turbo control.
EGR flow too low, often carbon or blockage related.
EGR flow too high, often stuck-open valve related.
Helpful for boost, EGR and fuel delivery symptoms.
Useful if diesel boost or EGR faults appear with DPF symptoms.
These codes can affect starting, cutting out, engine timing, misfires and whether the engine can run reliably.
VVT, oil control, cam timing, solenoid or timing chain-related fault.
Crank sensor fault that can cause crank-no-start, cutting out, stalling or no RPM signal.
Cam sensor, wiring or timing signal issue that can affect starting and running.
Useful if P0335 appears with no-start symptoms.
Practical checks for no-start faults.
Helpful for intermittent crank sensor and power-loss symptoms.
These codes can affect driveability, safety systems, charging, oil pressure and whether the car is safe to keep driving.
Speed signal fault affecting speedometer, ABS, traction control or gearbox behaviour.
High idle caused by air leaks, throttle body, PCV or idle control issues.
Oil pressure sensor or possible real oil pressure concern.
Battery, alternator, earth strap, charging or low voltage fault.
Useful if P0562 appears with charging warnings.
Useful if P0520 appears with oil pressure warnings.
P0700 is different from many engine codes. It usually means the transmission control module has detected a gearbox fault and asked the engine ECU to turn the engine light on. The real gearbox fault is normally stored inside the transmission module and needs a capable scanner.
Do not replace gearbox parts from P0700 alone. A proper scan should read the transmission module, check live data, inspect fluid condition and confirm whether the fault is electrical, hydraulic, sensor-related or internal.
The MOT tester does not normally fail a car simply because a scan tool shows a stored OBD code. The issue is what the fault causes. If the code causes an emissions-related engine warning light, poor emissions, visible smoke, ABS warning light, airbag warning light, serious oil leak, rough running, cutting out or safety issue, it can lead to an MOT failure.
⚠️ High risk: flashing engine light, misfire, emissions light, ABS/airbag warning, visible smoke, no-start or oil pressure warning.
⚠️ Medium risk: oxygen sensor, catalyst, EGR, EVAP, DPF, boost, crank/cam or fuel mixture faults.
✅ Lower risk: historic codes where the fault is repaired and no warning light or symptoms remain.
In real diagnostics, the cheapest repair is often the correct test. A P0171 may need a smoke test before a sensor. A P0420 may need misfire history checked before a catalytic converter. A P0299 may need boost pipes tested before blaming the turbo. A P0335 may need live RPM and wiring checks before replacing starting parts.
The best order is simple: scan, record data, check symptoms, inspect visually, test live data, check wiring, voltage and pressure, then replace parts only when the evidence supports it.
✅ Check battery voltage and charging system.
✅ Inspect intake pipes for splits or loose clips.
✅ Check for vacuum leaks and exhaust leaks.
✅ Read live data, not just stored codes.
✅ Check fuel trims for lean or rich running.
✅ Check live RPM while cranking for crank sensor/no-start faults.
✅ Check oil level and oil condition where relevant.
✅ Record freeze frame data before clearing codes.
✅ Road test after repair and check pending codes.
Find answers to common questions about fault codes, engine warning lights, diagnostic scans, safe driving advice and MOT implications for UK drivers.
An OBD fault code is a diagnostic trouble code stored by the car when a control module detects a fault. It helps identify the affected system, but it does not always confirm the exact failed part.
Not straight away. A fault code can point towards a system or sensor, but wiring faults, air leaks, low voltage, poor servicing, fuel pressure faults or another fault can trigger the same code.
It depends on the code and symptoms. A steady engine light with no running problem may allow short careful driving, but flashing engine lights, misfires, overheating, oil pressure warnings, brake warnings, no-start or loss of power should be treated as urgent.
The code itself is not usually what fails the car. The fault behind the code can cause an MOT failure if it affects emissions, warning lights, braking, ABS, airbag systems, visible smoke or vehicle safety.
Basic fault code readers are useful, but many only show engine codes and may miss ABS, airbag, gearbox or manufacturer-specific faults. A professional diagnostic scan can provide live data, freeze frame information and module-specific codes.
It is better to record the codes and freeze frame data before clearing anything. Clearing codes too early can remove useful evidence and make intermittent faults harder to trace.
