OBD Fault Code Lookup

Camshaft timing actuator circuit (bank 1)

The engine computer is not getting the electrical response it expects from the variable valve timing actuator on bank 1.

Commonly checked first

• Check the actuator connector for corrosion, damage or a loose fit
• Check engine oil level and condition — variable valve timing relies on clean oil at the right pressure
• Look for chafed wiring near the front of the engine

Camshaft timing over-advanced (bank 1)

Valve timing on bank 1 is reading further advanced than the computer commanded.

Commonly checked first

• Check oil level, oil condition and when the oil was last changed
• Check the timing actuator connector and wiring
• Note whether the fault appears only when the engine is warm or at certain revs

Crankshaft and camshaft position out of correlation

The crankshaft and camshaft sensors are not lining up the way the computer expects, so it cannot trust the engine's timing picture.

Commonly checked first

• Check oil level and pressure warning behaviour
• Check both sensor connectors
• Note any rattle from the front of the engine on a cold start

If this code comes with a rattle, rough running or a warning light, stop driving and arrange an inspection — timing faults can cause serious engine damage.

Ambient air temperature sensor range or performance

The outside air temperature reading looks implausible to the computer.

Commonly checked first

• Check the sensor behind the front grille for damage or debris
• Check the connector for water ingress

Fuel rail pressure too low

Fuel pressure is reading below what the computer expects while the engine is running.

Commonly checked first

• Check for a fuel filter that is overdue for replacement
• Note whether the problem appears under load, when hot, or at low fuel levels
• Look and smell for any sign of a fuel leak

If you can smell fuel strongly or see a leak, do not start or drive the vehicle. Arrange professional help.

Mass air flow circuit

The signal from the air flow sensor is missing or out of range, so the computer cannot measure incoming air properly.

Commonly checked first

• Check the air filter and the intake pipe between the filter and the engine
• Check the sensor connector is fully seated
• Look for splits in flexible intake hoses

Mass air flow range or performance

The measured air flow does not match what the computer expects for the current engine conditions.

Commonly checked first

• Check for a dirty, oily or wrongly fitted air filter
• Check intake pipework for loose clamps or splits
• Note whether the problem is worse on acceleration

Mass air flow circuit low input

The air flow signal is reading lower than the sensor's normal range.

Commonly checked first

• Check the sensor connector and wiring for damage
• Check for a blocked air filter

Intake air temperature sensor high input

The intake air temperature signal is reading higher than the expected range.

Commonly checked first

• Check the sensor connector for a poor contact
• Check the wiring where it passes near hot components

Coolant thermostat below regulating temperature

The engine is taking longer than expected to reach normal running temperature.

Commonly checked first

• Check the coolant level when the engine is cold
• Watch the temperature gauge on a normal journey and note how long it takes to settle

Oxygen sensor circuit (bank 1, sensor 1)

The signal from the front oxygen sensor is outside the range the computer expects.

Commonly checked first

• Check the sensor connector and wiring routing near the exhaust
• Listen for an exhaust leak before the sensor

Oxygen sensor circuit low voltage (bank 1, sensor 1)

The front oxygen sensor is reporting a persistently low voltage.

Commonly checked first

• Check for exhaust leaks near the sensor
• Check the sensor wiring for heat damage

Oxygen sensor slow response (bank 1, sensor 1)

The front oxygen sensor is responding more slowly than expected to changes in the exhaust.

Commonly checked first

• Check for exhaust leaks ahead of the sensor
• Note fuel consumption changes and any rough running

Oxygen sensor heater circuit (bank 1, sensor 1)

The heater element inside the front oxygen sensor is not drawing current the way the computer expects.

Commonly checked first

• Check the relevant fuse
• Check the sensor connector for corrosion

System too lean (bank 1)

The engine is running with more air than fuel compared with what the computer wants, and it has run out of adjustment.

Commonly checked first

• Check intake hoses and the oil filler cap and dipstick seal for unmetered air getting in
• Check the air filter
• Listen for a hissing sound with the engine idling

System too rich (bank 1)

The engine is running with more fuel than the computer wants.

Commonly checked first

• Check the air filter for restriction
• Note black smoke, fuel smell or heavy fuel consumption

System too lean (bank 2)

The same lean condition as P0171, reported on the engine's second bank.

Commonly checked first

• Check intake pipework and gaskets on that side of the engine
• Check whether a bank 1 lean code is present too, which points at something shared

Fuel rail pressure sensor circuit

The fuel pressure sensor signal is missing or out of range.

Commonly checked first

• Check the sensor connector on the fuel rail
• Check for chafed wiring around the engine bay

Injector circuit (cylinder 1)

The electrical circuit for the cylinder 1 injector is not behaving as expected.

Commonly checked first

• Check the injector connector clip is properly latched
• Check for damaged wiring along the injector harness

Engine over-temperature condition

The engine has recorded a temperature above its safe operating range.

Commonly checked first

• Check the coolant level only when the engine is completely cold
• Look for coolant staining or drips under the car

Switch the engine off as soon as it is safe to do so. Allow it to cool completely. Do not continue driving an overheating vehicle because serious engine damage may occur.

Random or multiple cylinder misfire

The engine is misfiring, and the computer cannot pin it to one cylinder.

Commonly checked first

• Note when it happens: cold start, idle, under load or in the wet
• Check service history for spark plugs due for replacement
• Check for a loose or damp ignition coil connector

A flashing engine warning light with a misfire means stop driving when it is safe — continued running can damage the catalytic converter.

Cylinder 1 misfire detected

The computer has detected misfiring specifically on cylinder 1.

Commonly checked first

• Check the coil and plug connector on that cylinder
• Note whether swapping parts between cylinders moves the fault — a common workshop test

Cylinder 2 misfire detected

The computer has detected misfiring specifically on cylinder 2.

Commonly checked first

• Check the coil and plug connector on that cylinder
• Note whether the misfire is constant or only under load

Cylinder 3 misfire detected

The computer has detected misfiring specifically on cylinder 3.

Commonly checked first

• Check the coil and plug connector on that cylinder
• Note any rough idle or vibration through the pedals

Cylinder 4 misfire detected

The computer has detected misfiring specifically on cylinder 4.

Commonly checked first

• Check the coil and plug connector on that cylinder
• Note whether the fault appears only in damp weather

Knock sensor circuit (bank 1)

The knock sensor signal is out of range, so the computer cannot listen for detonation properly.

Commonly checked first

• Check the sensor connector, often tucked under the intake
• Note any pinking or rattling under acceleration

Crankshaft position sensor circuit

The computer is not receiving a usable signal telling it where the crankshaft is.

Commonly checked first

• Check the sensor connector near the bellhousing or crank pulley
• Note whether the engine cuts out or refuses to start when hot

Camshaft position sensor circuit

The camshaft position signal is missing or implausible.

Commonly checked first

• Check the sensor connector and wiring
• Note any long cranking before the engine fires

Exhaust gas recirculation flow insufficient

Less exhaust gas is being recirculated into the intake than the computer expects.

Commonly checked first

• Note the type of driving the car does — lots of short trips contribute to soot build-up
• Check for obvious soot around EGR pipework joints

Catalyst system efficiency below threshold (bank 1)

The readings before and after the catalytic converter are too similar, so the computer thinks it is not cleaning the exhaust as well as it should.

Commonly checked first

• Check for exhaust leaks, which can imitate this fault
• Check whether any misfire or fuelling codes are also stored — those are usually addressed first

Catalyst system efficiency below threshold (bank 2)

The same catalyst efficiency reading as P0420, on the second bank.

Commonly checked first

• Check for exhaust leaks on that side
• Check whether other fuelling or misfire codes are stored

Evaporative emission system small leak

The fuel vapour system is not holding pressure, so a small leak is suspected.

Commonly checked first

• Check the fuel cap seal and that it clicks when tightened
• Check the filler neck area for damage

Evaporative emission system large leak

A larger leak is suspected in the fuel vapour system.

Commonly checked first

• Check the fuel cap is present, correct and sealing
• Look for disconnected vapour hoses near the charcoal canister

Vehicle speed sensor

The road speed signal is missing or implausible.

Commonly checked first

• Note whether the speedometer behaves normally
• Check the sensor connector at the gearbox or wheel hub

Idle air control system

The engine's idle speed control is not achieving the target idle.

Commonly checked first

• Check for intake leaks
• Note whether the idle hunts, drops or stalls

Engine oil pressure sensor or switch circuit

The oil pressure signal is missing or out of range.

Commonly checked first

• Check oil level on level ground with the engine off
• Note whether the red oil pressure light behaves oddly

If the red oil pressure warning light comes on while driving, stop the engine immediately when safe. Do not keep driving.

System voltage low

The electrical system voltage is reading below the expected range.

Commonly checked first

• Check battery terminals for tightness and corrosion
• Check the earth strap between engine and body
• Note dimming lights or slow cranking

Engine control module processor fault

The engine computer has reported an internal problem with itself.

Commonly checked first

• Check battery condition and charging voltage first — low voltage causes false module faults
• Check module connectors for water ingress

Transmission control system fault

The gearbox control module has stored its own fault and is telling the engine computer about it.

Commonly checked first

• Note the exact gearchange behaviour and when it happens
• Check gearbox fluid service history where the vehicle has a serviceable fluid

Input or turbine speed sensor circuit

The gearbox cannot read its input shaft speed reliably.

Commonly checked first

• Note whether the gearbox holds one gear or changes harshly
• Check the sensor connector at the gearbox

Incorrect gear ratio

The gearbox is not achieving the ratio it selected, based on input and output speeds.

Commonly checked first

• Note which gears are affected and whether it slips under load
• Check fluid level and condition if the gearbox has a check procedure

Transmission fluid pressure low

Internal gearbox pressure is reading below expectation.

Commonly checked first

• Look for fluid leaks under the gearbox
• Note slipping or delayed engagement

Readiness monitors not complete

The car has not finished running its self-tests yet, often after a battery disconnection or a code reset.

Commonly checked first

• Drive normally for a mixture of town and faster roads and re-read the codes

Diesel particulate filter efficiency below threshold

The particulate filter is not performing as expected according to the sensors around it.

Commonly checked first

• Note the driving pattern — frequent short trips prevent the filter regenerating
• Check for repeated warnings after cold starts

Post-catalyst fuel trim system too lean

Readings after the catalytic converter suggest a persistently lean mixture.

Commonly checked first

• Listen for an exhaust leak between the engine and the sensor
• Check whether other fuelling codes are stored alongside

System too lean at idle (bank 1)

The lean reading appears specifically at idle.

Commonly checked first

• Check intake hoses, the oil cap and the dipstick seal
• Listen for hissing with the engine idling

Diesel particulate filter soot accumulation

The particulate filter is reporting more soot than expected.

Commonly checked first

• Note how the car is normally driven
• Check for repeated short journeys that interrupt regeneration

Lost communication with engine control module

Another module has stopped hearing from the engine computer on the car's network.

Commonly checked first

• Check battery voltage and terminal tightness
• Check for water ingress in connectors and fuse boxes

Lost communication with ABS module

Communication with the anti-lock braking module has been lost.

Commonly checked first

• Check battery condition and earth connections
• Note which warning lights are showing together

If braking feels weak, the pedal travels further than normal, or a brake warning light is on, avoid driving and arrange a professional inspection or recovery.

Engine control system fault reported to ABS

The braking or stability system has recorded an engine-side fault affecting its operation.

Commonly checked first

• Note every warning light currently showing
• Check whether engine codes are stored at the same time

If stability or braking behaviour has changed, treat it as a safety fault and arrange a professional inspection.

Body control module fault

The body control module has recorded an internal or circuit fault.

Commonly checked first

• Check battery voltage and terminals
• Note any recent electrical work, accessories or water ingress