Help!!! Electrical gremlins
Thread Starter
4th Gear
Joined: Jun 2026
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Help!!! Electrical gremlins
Randomly, my climate controls, reverse lights, and O2 sensors stop working. But then they start working again! Sometimes for a week, sometimes for a day... sometimes they come back on while driving but never shut off until I turn the car off. I have cleaned all the grounds and had it on the lift; everything looks good—no bad plugs or wires. Fuses and relays are good. I don't know, does anyone have a good guess or a crystal ball?
I have pulled all connectors, fuses, relays, and grounds. I haven't checked voltage yet, and I know I must. I just wanted to see if there is any known issue causing this.
I have pulled all connectors, fuses, relays, and grounds. I haven't checked voltage yet, and I know I must. I just wanted to see if there is any known issue causing this.
Last edited by Ryanlema117; Today at 01:59 PM.
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Joined: Jul 2010
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Randomly, my climate controls, reverse lights, and O2 sensors stop working. But then they start working again! Sometimes for a week, sometimes for a day... sometimes they come back on while driving but never shut off until I turn the car off. I have cleaned all the grounds and had it on the lift; everything looks good—no bad plugs or wires. Fuses and relays are good. I don't know, does anyone have a good guess or a crystal ball?
Last edited by helga203; Today at 12:14 PM.
Tech Contributor
Joined: Jan 2007
Posts: 19,525
Likes: 1,195
From: Dyer, IN
Randomly, my climate controls, reverse lights, and O2 sensors stop working. But then they start working again! Sometimes for a week, sometimes for a day... sometimes they come back on while driving but never shut off until I turn the car off. I have cleaned all the grounds and had it on the lift; everything looks good—no bad plugs or wires. Fuses and relays are good. I don't know, does anyone have a good guess or a crystal ball?
Thread Starter
4th Gear
Joined: Jun 2026
Posts: 4
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I'm not expecting that to fix the issues. I am trying to troubleshoot. I will admit my automotive knowledge is lacking. I understand the basics of a 12v system on my 76 luv and 82 amc its easy , but sadly I am an Industrial Tech. AC is my bread and butter and newer cars have me lacking but I am here to learn, so the shop is a last stop. Now, if you were like, "Oh, yeah, try this," that would be helpful because anyone can go to a shop, haha. I don't have a big-*** garage and two posts to just park my other cars between for fun.
Checking for voltage is the next step, but it seems to work every time I have time to put it on the lift.
Checking for voltage is the next step, but it seems to work every time I have time to put it on the lift.
Thread Starter
4th Gear
Joined: Jun 2026
Posts: 4
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You don't need a crystal ball to learn how to properly diagnose your problem. What you told us is not troubleshooting. Shining a flashlight on wires and components is not going to fix your problem. Sound like you have limited troubleshooting skills. Take it to a electrical shop. Not ripping on you. These cars take people with some high degree of electrical knowledge.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
Well intermittents are the hardest to diagnose...the electrical systems on a 70 or 80's car is the same as on your C5 and this Forum isn't the place to "learn"...we can give you direction and maybe even guess for you correctly but that's about it...how do you know your O2 sensors stopped working ???
Last edited by C5 Diag; Today at 02:44 PM.
Tech Contributor
Joined: Jan 2007
Posts: 19,525
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From: Dyer, IN
The troubleshooting is the same. You identify the impacted systems, looking for common denominators....in this case for your problem, they all are fed from the line side of one contact in the ignition switch. Like troubleshooting AC, you will need to understand schematics........no different on a car. DMM and test light.....no biggie.
Just bear in mind this a forum that deals with specifics related to C5 issues......not a "how to work on cars" forum. 
Also fill out your profile with what year C5 you have...they are all not the same.
Also fill out your profile with what year C5 you have...they are all not the same.
Last edited by lucky131969; Today at 02:24 PM.
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Thread Starter
4th Gear
Joined: Jun 2026
Posts: 4
Likes: 0
Well intermittents are the hardest to diagnose...the electrical systems on a 70 or 80's car is the same as on your C5 and this Forum isn't the place to "learn"...we can give you direction and maybe even guess for you correctly but that's about it...how do you know your O2 sensors stopped working ???
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,666
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From: Cape Coral, Florida
Normally with an ignition switch issue you will see a common feed for all circuits and in your case I don't think your HVAC, BU lights, and O2 sensors share this common feed...if you want to check the HVAC place one lead of your DMM on battery positive and the other lead on fuse 27 at the IP fuse block...now read the voltage drop when it is acting up...if your DMM reads 3 volts you have a 9 volt drop across the ignition switch...now do the same with fuse 21 at the IP fuse block...for your O2 sensor it produces its own voltage when hot and the car is running so what DTC do you have ??...don't think it's related to the other two...with the sensor cold and fuse 15 removed you should see a 450mv "bias voltage" sent out by the PCM with the ignition ON.
Tech Contributor
Joined: Jan 2007
Posts: 19,525
Likes: 1,195
From: Dyer, IN
Normally with an ignition switch issue you will see a common feed for all circuits and in your case I don't think your HVAC, BU lights, and O2 sensors share this common feed...if you want to check the HVAC place one lead of your DMM on battery positive and the other lead on fuse 27 at the IP fuse block...now read the voltage drop when it is acting up...if your DMM reads 3 volts you have a 9 volt drop across the ignition switch...now do the same with fuse 21 at the IP fuse block...for your O2 sensor it produces its own voltage when hot and the car is running so what DTC do you have ??...don't think it's related to the other two...with the sensor cold and fuse 15 removed you should see a 450mv "bias voltage" sent out by the PCM with the ignition ON.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,666
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From: Cape Coral, Florida