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1999 Chevy Silverado 1500 O2 sensor problems

SgtMarshal

SgtMarshal

Active member
my friend has a 1999 Silverado with the 5.3 engine. he has been having issues with it lately. I scanned for codes, and there is a P0131, P0137, P0157, and a P0151, which are all related to front and back O2 sensors. there is also a P0300. my friend doesn't have money to pay a mechanic and so I'm trying to help him. I am not the best mechanic. I figured I would start on it tomorrow and check for any exhaust leaks. the manifolds are oem. I did notice on my scan that the LTFT is +25, and the O2B1 on the hp tuner chart is more wavy than my grand prix, like it isn't changing voltage very fast. I'll have to add all the other O2 sensors to read what they way.
 


being all those codes are low voltage. id start looking for a blown fuse. pretty sure that trucks got a under hood fuse box and one in the d/s dash side, open the door pull the cover off deal.
 
I was wondering about that, I looked on the passenger side and didn't see anything, I'll check like you said.
 
I checked the o2 sensor fuses and they look good. I unplugged the mass air flow sensor and the fuel trims dropped to 0, and started to rise as long as there was no throttle applied. with a little throttle, whatever they were at before, they would come within +5 of 0.

I will take it for a spin around the block tomorrow and see how the truck does without the maf plugged in. I think if the maf reads wrong, then the o2 sensors are all going to be affected like they were.
 
Just know the IAT in built into the MAF on these truck, nothing is going to look right when it says the intake temps are at -40 all the time. It will throw everything for a loop, it will command more fuel, different timing, it throws closed loop and open loop functions off too.
 
I plugged the maf sensor back in and cleared the codes, and I couldn't get any codes to show up. the only problem I could see was that both banks of fuel trims were +25 most of the time. I took the truck back to the owner, and on the drive over, I went full throttle and the truck fell on its face. I let off and the truck continued driving like normal. this problem is definitely above my ability level to diagnose and repair.
 


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