A check engine light with a 'heater circuit malfunction' usually indicates a failed oxygen sensor in your vehicle. A failed oxygen sensor may also produce a weak idle, stalling, erratic jerking, and poor gas mileage. The oxygen sensor monitors the amount of fuel that enters the engine. Before you replace the sensor or schedule automotive repair, test it first to find out if it is the problem.
Get Ready to Test the Sensor
Raise the hood, and locate the oxygen sensor, and refer to your manual, if needed. On models made before 1996, the sensor should be located near the exhaust manifold. Many new vehicles commonly have two oxygen sensors: one installed close the engine manifold and the other installed near the catalytic converter. You will need to raise newer models on floor jacks and support the wheels with jack stands to check the sensor near the catalytic converter.
Visually inspect the sensor for cracks and damaged wires, which should be curved or straight. Move wiring away from exhaust components. If everything looks good, proceed to test with a voltmeter.
Test with a Voltmeter
Let the vehicle run ten minutes to help heat the sensor for an accurate reading. Shut off the motor, and set the voltmeter on the DC mode.
Carefully attach the positive red lead from the voltmeter to the back probe of the sensor's signal wire and the black probe to the chassis (lower frame) of the vehicle to read voltage from the rear. If the voltmeter lacks a back probe back probe, gently detach the sensor connector,= connect a small jumper cable to the signal wire prong, then reinstall the connector. D0n't touch the engine, which will still be hot.
Set the voltmeter to 1, and check the voltage which should fluctuate between .0.1 and 1.0 volts. Start the vehicle and watch for the voltmeter to change rapidly. If the voltmeter doesn't change, the sensor is faulty.
Do a Vacuum Leak or Propane Enrichment Test
If you got a positive result, test how the sensor reacts to lean-fuel air mixture. Your engine usually has a vacuum port on top., which when you open it, creates a vacuum. The meter should produce a reading under 0.1 after you open it, and when you close the port, it should not read above.05.
Do a propane enrichment test if your engine doesn't have a vacuum port by adding a small amount to propane fluid to the air intake. The addition of fuel should cause the voltmeter to rise rapidly. If the voltmeter remains unchanged or decreases, the sensor needs replacing.