sc300jon

How do you test an oxygen sensor? Or do mechanics just look for diagnostic codes to indicate when one is not functioning properly?

-Jon
'92 SC300

SC300T

The book mentions something about measuring the voltage output by the sensor while the car is running. After warmup, the sensor output should fluctuate from low to high voltage within a certain time frame, the faster the engine speed, generally the faster the fluctuation. You put the Volt meter on some ECU leads to check it. Around idle, you may only get one swing within three seconds, but under load and say 3k rpm, it should swing quickly from low to high, perhaps one oscillation per second. I forget where how much voltage is deemed "stoichiometric" or the ideal mixture, but I think it was around .7 volts... When they go bad, they get "lazy" and don't react as fast. They'll also not fire up as quickly when the engine is cold. (Non-heated 02 sensors don't send a signal when the engine is cold --- It may take a minute or two in cold weather). My heated 02 sensor takes about 10 seconds to come to life. Totally bad 02 sensors will cause a malfuntion light to eventually come on, but even if you disconnect the 02 sensor, it may take a while for the computer to illuminate the check engine light. A lazy 02 sensor will not illuminate the check engine light, and probably wounldn't affect drivability other than a slight reduction in gas mileage.

DevilDog

First you need a digital graphing volt meter. If you open the check connector under the hood. You'll see two pins, labeled OX1, and OX2. That's where you tap in with your meter. Positive lead to the pin connector, negative to ground. Run the engine at about 2,000 rpm and watch the voltages sweep from .2 - .9 volts, up then down continuously. That is the basics. Anymore and I'd have to show you myself.

Did someone tell you they were bad? Did the check engine light come on? If so was it a 25, or 26 code?

sc300jon

I don't know if they are bad or not. The idle is rough at around 750 rpms (manual trans in neutral).

Somebody mentioned that will 94k on the O2 sensors it could be time to replace them. I thought I would at least try to check them.

How to I read the diagnostic codes? I assume I need a special reader, right?


-Jon