What Makes a Service Engine Soon Light Come on?

by Edmund Gary
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Modern cars have onboard computers that examine the emissions levels of the exhaust. This crucial system is needed to to keep the car running within the requirements of the environmental laws of the local and state and national jurisdictions. Whenever the "service engine soon" light comes on steadily, it is best to take the vehicle to a certified technician so the problem can be properly corrected.

A Diagnostic Tool

Every time the car is turned off and restarted, the engine control module (ECM) and power train control module (PCM) perform a series of checks of the sensors and components of the car, according to TrustMyMechanic.com. The service engine soon light comes on while the car is starting and should go out while the engine is running.

Oxygen Sensor Related Problem

The ECM or PCM may have detected a P0133 code (code in a scan tool for oxygen sensor related problems). According to Samarins.com, the oxygen sensor in front of the catalytic converter is responding slowly to changes in the air/fuel mixture. This could be caused by a problem with the wiring, an exhaust leak, bad airflow sensor or a bad oxygen sensor.

Stuck Thermostat

A thermostat stuck in the open position can cause the "Service Engine Soon" lamp to come on, according to 2CarPros.com. The coolant stays at a low temperature and this will cause the ECM or PCM to stay in an open loop. This open loop creates a state of pre-programmed parameters in which no diagnostic tests can be executed.

Shorted Fuel Injector

The fuel injector of the modern car is electronically controlled. In some cases, the fuel injector may have a short circuit. This will cause the engine not to run, according to 2CarPros.com. This can cause the injector driver in the computer to short out. This may lead to incomplete code gathering properties, which results in a P1000 code (code in a scan tool for shorted out fuel injector).


The service engine soon light comes on to alert the driver of a potential problem. The check engine light is primarily a monitor of the emissions system, according to ShadeTreeGarage.com. The car can still be driven. Once the light comes on while driving, it is best to have the problem diagnosed and repaired at the earliest convenience.

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