The other day I turned my a/c on for the first time this year. After about 20 min, the check engine light came on. My husband checked it and it showed an O2 sensor, so he cleared it. The O2 sensor was changed less than 1000 miles ago. Now today, I turned the a/c on again, after about 10 min, the light came back on. The a/c is working great, cold air, car running great, no problems. Any suggestions. The first time this happened the car was sitting idol with the a/c running. Should the code be checked and cleared with the a/c running, because the first time he did it, the a/c was turned off. Thanks for all your help. This is a great site.

If you can tell us what year & what model vehicle you have maybe we can help you from years to years O2 sensor from one sensor up to four sensors check engine light has nothing to do with A\C on or off.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner

It is a 2000 sl500. Two O2 sensors have been replaced. I thought it was weird too about the a/c, but that is the only thing different and both times, the light came on after the a/c was turned on. Thanks for your help. You guys have helped me before and were right on.

Your car has four O2 sensors. It's possible that another one has failed. Checking the trouble codes will tell you which one to check.

Would the a/c affect that. Today, the car was fine since the codes were cleared again last night. The a/c was on, set at 72 but didn't seem to be running as cold as it should have, so I pushed the auto control to set it to 70 and as soon as I did that, the check engine light came on again. I don't think its blowing as cold as it should. I didn't think that O2 sensors had anything to do with the a/c.