1991 GMC K 3500 O2 sensor/check engine light trouble

**afish1** · 09-03-2009, 08:03 PM

This is my first post so first off hello to everyone. I purchased a 91 GMC K3500 with a 5.7 I purchased it with the check engine light on. The seller said it was 02 sensor. It needed alot of work so I brought it home and fixed it put a TON of new parts including a brand new 02 sensor. Well now that I got it going again the check engine light still goes on and off. It will be off when I first start the truck but after 5-10 minutes of driving it will come on. If I go into a store for a while and come out it will be off again until I get going again I think it turned off by itself while driving but not positive on that since I have only driven it once. THe truck sat in my garage for a couple of months with no battery so I know the code should be cleared. I stopped at Murry's and the code scan said it was the O2 sensor but its brand new so what else could be causing these troubles without it actually being the O2 sensor. I only saw one sensor before the cat and and the O2 sensor has a single wire. I hope I have provided enough info and thank you in advance.

**dsfloyd** · 09-03-2009, 08:42 PM

maybe not on that old of a model but there is usually one after the cat as well, which may be turning on the light once it gets hot.

**silveradotrailblazer** · 09-03-2009, 09:37 PM

Trucks built before 1996 have only one O2 sensor, infront of cat. Some of the early GMs had trouble with the O2s not getting hot enough to work, so they have a heated unit for this. Some of the early trucks had the O2 sensor in the exhaust manifold and this did not let the O2 get hot enough so GM moved the O2s down closer to the cat so the O2 would get hotter. If you truck has headers this can keep the O2 from getting hot also. Alot of guys that remove or gut the cat have to run a heated O2 sensor. Hope this will help.

**afish1** · 09-04-2009, 10:11 AM

Yes I only have one o2 sensor right in front of my cat. I looked for a heated unit for my year and came up empty. Can I just use a 4 wire o2 out of a newer truck and rewire it? Will my ECM be OK with this or will I need to do something else (have it reprogrammed) the truck is stock no Headers (its actually a dump truck) so no performance mods by me and none I can tell from previous owner. As far as the cat goes its still there. How do I tell if its been gutted or not? I was reading through my Chilton manual and they said a vacuum leak or a malfunctioning pressure regulator can cause extreme conditions that the o2 sensor cannot compensate for. Anyone have experience with this? I know my fuel vapor canister has a small crack in it. Could this be causing my trouble ? If so can I bypass it without having any issues? Or at least temporarily bypass it to see if that is the problem ? There are two hoses that connect to it one to the TBI and one to the gas tank Can I just disconnect them and cap them or should I hook them together for the test. I'm not sure what chiltons was referring to when they said pressure regulator (Did they mean fuel pressure regulator) ? If so I'm not sure how to go about diagnosing the Fuel pressure regulator.

**sgui** · 09-05-2009, 08:48 AM

what is the exact code does it give

**2COR517** · 09-05-2009, 10:11 AM

Did the truck originally have a one wire sensor? You could also have a wiring problem, so the O2 signal is not coming up.

How does the truck run?

**afish1** · 09-05-2009, 01:53 PM

Sorry not sure of the exact error code I went to murrys and they had just reopened as Orilly auto parts all new employees they didnt seem too knowledgeable I was kind of counting myself lucky that they where able to hook up the scanner and find the obd1 port. Just saw it said O2 sensor. I will try to take it up to autozone. Yes it originaly only had a 1 wire sensor.