SES light help

Mekanic · April 30, 2014

Hey guys, new here but not to my truck, 99 silverado z71 extended cab, bought brand new, has about 168k on it now. Aside from u-joints, an AC compressor, and that damn quick disconnect on the heater core, it's been bullet proof.

I'm having a weird issue. driving home today, my SES light came on, no odd smells or sounds, no lack of power, everything felt fine, all gauges reading normal, so I drove on home, figuring it was an IAT sensor or something silly.

Tonight I went to put my scan tool on it (autotap) and the connected light on it was very faint, flickering,(autotap has a red LED that illuminates when plugged into an OBDII port) and it could not make a connection to the ECM.

Then I noticed, that when my scanner was plugged in, it caused the ABS light to come on, and the SECURITY light to come on on the DIC. Puzzled I put my autotap on the wifes Jeep to confirm it wasn't a problem with the tool, and it worked perfectly there. I also checked the connections on all the pins on the back of the port and none were loose, blew it out a bit, will try some deoxit on it if that may be where the problem lies.

I'm positively stumped on this one. ECM going bad? Bad ground? I have no clue.

Any help is IMMENSELY appreciated.

~Rich

Mekanic · April 30, 2014

UPDATE.

A MASSIVE thank you to txab for your knowledge. I found a post of his from 2009 when looking for info about a bad PCM or BCM, and he said if you had trouble getting the codes, check the cigar lighter. I then remembered that when helping someone roadside and using a 12v compressor about a month ago, I blew the fuse for the lighter.

Just replaced fuse, read the codes (knock sensor, probably from noisy replacement AC compressor) cleared and all is now right again.

I would have NEVER guessed that the frigging lighter fuse would be tied into the OBDII port.

Thanks again txab, you saved my butt without even posting haha.

~Rich