Hello Paul, my Toyota Noah with a D4 engine is showing the check engine light. We have replaced the air cleaner, spark plugs and done an engine oil service. The light will not go off. What could be the problem? Khauka
Hello Khauka, a check engine fault warning light on your dashboard is a result of an emission-related fault. The key points to investigate are ignition system, air intake, fuel system, vacuum system and engine management. Ignition system includes spark plugs and ignition coils.
Air intake includes the air filter, intake hose and throttle unit. The fuel system includes the fuel pump, filter and fuel injectors while the vacuum system will be the small vacuum hoses around the engine.
The engine management system is better inspected using a computer diagnostic tool to confirm that none of the vital information relaying sensors (oxygen, air flow, coolant, crankshaft, camshaft, fuel pressure, throttle) are faulty.
An alteration of spark, fuel or air ratio will affect the quality of burning while a drop in the performance of any of the engine management sensors will cause a check engine fault.
The diagnostic computer will pick a fault code which will guide the technician to determine which of the above components is not working well. If your Noah has made 100,000kms, consider replacing the long life fuel filter and spark plugs with genuine parts.