I know this topics been discussed to death even more than what tire size you can fit, but my Check Engine light has been on for over a year. Throwing a PO171 and PO174 (Lean bank one and bank two) on our trucks what is the most common cause of this?? I've heard everything from o2 sensors to vacuum leaks. Also my front fuel pump is a little on the weak side which would cause it to run lean, but I don't use that tank often. Do our trucks rescan intermittently or do they have to be manually cleared??

 

I do know the obd1 system will retain codes in memory. I might be wrong but it doesn't seem to function like obd11, performing system tests. Seems to be continuously monitored in a way, when a sensor falls outside its set parameters a cel is set. On my truck when I was having egr issues upon startup the cel would be off until system went to closed loop. At that point the cel would return. Seems like It had to test a sensor as bad each time I drove it to set off the cel.

 

My truck is a 96 and it has OBDII and the light is always on. Now that I think about it my exhaust is cut off under the cab about an inch past the EGR tube. Could excess airflow up the EGR tube cause it to run lean??

 

Ummm, the EGR tube comes off of the RH exhaust manifold. I hope your exhaust isn't cut off that far up!!! A failed o2 sensor usually makes the vehicle run rich. It still may be the problem but I would try to find a vacuum leak

 

The EGR tube is right after the cat if I'm not mistaken?? If not what is that tube?? It's about an inch in diameter. I haven't been able to actually crawl under and inspect anything in a long time, seeing as we have close to a foot of snow on the ground still.

 

That's an air tube from the air pump. It pumps fresh air into the cat to make it burn hotter. Not the EGR. That's a really short exhaust you have there???? You should probably run some pipe and dump it back by the axle at the least.

 

I see. I thought the one on the manifold was the AIR tube. And just a bit. I caught the resonator on a tree stump and mangled the whole exhaust from there on back. So I hacked it off an called it a day.

 

A dirty MAF can cause those codes too.

 

Somebody, possibly you now that I think about it, suggested that when the CEL first came on. I cleaned it and reset the code and within five minutes of running it returned.

 

These trucks don't have a resonator. They have 2 cats. That's why the air tube is after the first one. It warms the second. The first cat is a different design and warms more easily so it cleans the air at start up. During start the air system pumps air to the headers or heads depending on design and burns the extra fuel from the engine running rich at start ( like the choke on a carb ). After a few seconds it switches to the rear tube to heat the second cat. Once in closed loop and at operating temp the air switches to vent to the atmosphere and the second cat does most of the work. The first cat does not control NOx as it isn't produced until the exhaust gets hot.
These codes aren't lean codes. 171 reads right bank or rear 02 sensor switching slow and 174 says it is not responding. Depending on year this may be a missing sensor. The rear sensor is after the cats to monitor cat performance. I believe these are on 93 and later models. Someone feel free to correct me on that. If your missing your exhaust the sensor went with it making it hard for the ecu to communicate. If you have a single sensor system then you may have a bad sensor. Its likely to be a problem with the sensor or wiring because it isn't stuck in a particular mode ( rich or lean). If you have a lean condition the fuel system will go far rich to try to fix it and the sensor will keep moving. In this case its not moving. To verify have a shop check the fuel trim for you.