BMW check engine light on
The check engine light (an engine outline, "Service Engine Soon" on older US BMWs) means the engine control module found a fault that can affect emissions. A steady light needs checking soon; a flashing light means misfires that can damage the catalytic converter, so ease off and have it checked right away.
A generic OBD2 code reader shows the emissions code (a P-code). The engine module stores more: BMW's own codes, with the conditions when each fault happened, which a tool that speaks BMW's protocols reads.
Usual causes
- A loose or worn gas cap, or a small leak in the fuel tank's vapor system.
- Misfires: ignition coils, spark plugs or fuel injectors.
- Oxygen sensors, or their heaters.
- Vacuum leaks: hoses, the crankcase vent valve, the intake.
- The secondary air pump and its valve on many 1990s and 2000s BMWs, mostly after a cold start.
- The VANOS (variable valve timing) or its sensors.
What to do
- Read the faults in the engine module, BMW codes and P-codes: the code and its details point to the part.
- Fix the cause, then clear the faults. Clearing also resets the readiness monitors, so drive the car before an emissions inspection.
- Check the readiness monitors before an inspection: the OBD readiness how-to shows how.
General guidance from how BMWs work, not a diagnosis of your car: the fault the module stored, with its details, tells you more.
The fault codes behind it
- Engine electronics fault codes 5694 codes · E46, E52, E53, E60, E61, E63, …
- Diesel electronics slave fault codes 1671 codes · E65, E66, E67
- Transmission control fault codes 789 codes · E53, E60, E61, E63, E64, E65, …
How-tos
Reading the fault
AutoLingo for BMW reads the fault behind the light from the module that stored it, in plain English and with its details, and clears it after the repair, with BMW's own diagnostic protocols: a generic OBD2 code reader only reaches the engine's emissions faults. See what it does on your BMW.
Questions
- What does a flashing check engine light mean?
- The engine is misfiring badly enough to damage the catalytic converter. Reduce speed and load and have it checked as soon as you can.
- Can I pass a smog check right after clearing the light?
- Usually not: clearing the codes resets the readiness monitors, and most inspections need them completed. Drive the car for a few days first and check the monitors.
- Why does my code reader show nothing while the light is on?
- Some BMW faults are stored as BMW codes, not P-codes, and a generic code reader doesn't show them. A tool that speaks BMW's protocols reads both.
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