Hi everyone, I'm getting a check engine light that's pretty much constant on my 1989 307 powered Cadillac and was hoping I could get some help with it.

It'll first come on when the engine is started cold or cool and stays on until I cut the ignition. Depending on how long I had been driving, it will stay off the next time I start the engine. For example I left my girlfriend's house this morning, it was about 72 degrees outside and the light came on a few seconds after startup and stayed on for my whole ride home, about half an hour of city and highway driving. Left for work a little while after, no light at all during my 20 minute ride to work. Looking at a different example, on a cold start the check engine light stayed on the entire time I drove for about 15 minutes. Parked the car for a little while and restarted it with the light back on the entire time.

I checked the codes and I'm getting 13: O2 sensor open circuit, 21: TPS circuit signal voltage high, and 34: pressure sensor circuit voltage out of range.

What should I check for? I'm not getting any unusual behavior from the engine or anything else in the car, it runs and drives pretty much normally. The only thing out of the ordinary would be a bottle of Lucas fuel system cleaner I added to the fuel tank since the car sat for a few weeks due to the weather being really hot and humid. I run the car on premium fuel and have for the last few months.

Thanks, hope I can get this taken care of soon. I'd like to take the car on a road trip next month.

 

I would start with the 13 since it indicates an open circuit. Get the wiring diagram and find out where the circuit is open. The other codes may be related but that one should be the easiest one to fix first. Alldata has a subscription service where you can get a subscription for one car for a year for about $30 You almost have to have it to troubleshoot any newer vehicle.

By the way, running the car on higher octane fuel than what it needs can actually be more harmful than good. My Wifes 84 Riviera has the same basic engine and it has gone almost 230K miles on 87 octane and has run just fine.

You should use a good brand of fuel. I am partial to BP and Shell.

 

Yeah I'm pretty partial to BP myself. I tend to stay away from the cheaper brands like Delta and US Gas. I talked to a tech at work (I work for a dealership) who has a lotta experience with this vintage of GM cars and suggested a bad ground on the O2 sensor. Since it grounds on the pipe it screws into it makes sense when compared to what I'm experiencing. Cold pipe, bad ground, check engine light. Pipe heats up and expands, ground connection made, no light.

Sounds easy right? Famous last words though......