During the diagnostic test procedure for a Catalyst Efficiency code, a diagnostic scan tool is connected to the DLC with the vehicle at operating temperature, and the data screen displays the pre-catalyst oxygen sensor continually switching between 200 mV - 800 mV, while the post-catalyst oxygen sensor's output voltage stays relatively steady at 700 mV. Technician A says that the post-catalyst oxygen sensor is fixed rich and should be replaced. Technician B says that the oxygen sensor readings are both normal. Who is correct?
A) A only
B) B only
C) both A and B
D) neither A nor B

Question

Quizplus · Accepted Answer

Both oxygen sensor readings are normal. The pre-catalyst oxygen sensor should switch between high and low voltages while the post-catalyst oxygen sensor should have a consistent voltage. A consistent voltage on the post-catalyst sensor indicates that the catalyst is doing its job of cleaning up the exhaust gases. The post-catalyst oxygen sensor being "fixed rich" is not a valid diagnosis.

During the Diagnostic Test Procedure for a Catalyst Efficiency Code