What Your Check Engine Light Means

February 15th, 2016

There it is, right there on your dashboard display…the Check Engine light. The light by itself doesn’t tell you much, does it? A lot of customers that we see at Auto Repair in Nashville TNAmerican Tire Company aren’t sure what it even means, so we’d like to give a little explanation.

Prior to the late 80s or early 90s, engine functions like fuel metering, fuel mixture, ignition, spark advance and transmission shift points were governed by mechanical systems and/or vacuum. Today, that’s all accomplished by the engine control computer, or powertrain control module (PCM). The PCM is what enables a V8 engine to deliver excellent performance with low emissions and surprising fuel economy.

The PCM relies on readings from a series of sensors and switches on the engine or in the exhaust stream. Some of these sensors include:

·         Oxygen sensors: located on the exhaust pipe or near the exhaust manifold, the O2 sensor monitors the fuel mixture by detecting the amount of unburned fuel in the exhaust stream. The PCM uses this information to readjust and continually fine-tune the engine’s air/fuel ratio. O2 sensors do need to be replaced as needed in the life cycle of a vehicle.

·         Coolant sensor: this monitors the engine’s operating temperature. The PCM takes temperature info and uses it to regulate fuel metering and ignition. For instance, like on an older carbureted engine, the fuel mixture needs to run a bit richer while the engine is still cold. When it warms up, the PCM will rely on the signal from the O2 sensor to “lean-out” the mixture again.

·         Throttle position sensor: the PCM monitors the load (acceleration, hill climbing, etc) on the engine through the throttle position sensor and the manifold absolute pressure sensor, which reads the amount of air being sucked through the intake manifold


Other sensors such as the crank position sensor, knock sensors and vehicle speed sensor all send information to the PCM, which it uses to govern engine functions. When the readings from any of the sensors is out of normal specs, it will register a “trouble code” in the PCM and illuminate the dashboard Check Engine light. A technician can then connect a code reader device to a diagnostic connector and quickly determine the origin of the trouble code.

The trouble code isn’t the be-all, end-all for diagnosis, as the underlying cause for the bad reading from a sensor may not be readily apparent. The technician has to then figure out why the trouble code has been triggered, but it certainly takes a lot of the guesswork and “gut feeling” out of troubleshooting on an engine.

NOTE: a flashing Check Engine light could be a sign of a real problem. On many vehicles, it means that the engine has a severe misfire that’s dumping raw gas into the exhaust system, which could quickly ruin the catalytic converter and result in a serious auto repair bill.

We hope that clears up any confusion you might have about the Check Engine lamp. Is yours illuminated? Make an appointment with us at American Tire Company so we can do a quick diagnosis on it!




  Posted in: Auto Repair 101