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Testing an O2 Sensor


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Hi all:

I read the following as part of a diagnostic approach:

"Next step is to measure the voltage at the oxygen sensor. The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it."

My question is, how do I test the voltage - with the engine tunning, or just the ignition on? Which wires do I tap on a heated O2 sensor, etc., etc.



1996 El Dorado

2006 STS

2000 Corvette

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I'm surprised it said the voltage should be low and steady. Maybe FI systems are just that good today. From what I understood (and observed) with conventional O2 sensors on older vehicles is that the O2 sensor's voltage would bounce back and forth between a low mV (100-200) and a high mV (700-800) repeately. As the system went lean and rich, it "evened out" in the middle.

Maybe today's systems are just that good and can run a pretty steady stream.

The measurement should be taken while the engine is running.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Check out your service manual. On my 93 Northstar powered Allante, I was able to go into the onboard computer and monitor each O2 sensor while driving down the road. The bad sensor just read 0 while the good one varied. This test is performed while driving and with the engine warmed up.

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Yes, the O2 sensors swing from low to high and back all day long.

His service manual test is for the O2 sensor after the Cat. The Cat 'scrubs' any remaining exhaust gas.....so the after Cat O2 sensor should always display low voltage. If it does swing in voltage like the front ones....the the Cat is not doing its job.

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