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DIL's O2 sensor problem - SOLVED

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Basil

Basil

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My DIL's 2004 Grand Prix was throwing codes indicating a bad O2 sensor and O2 sensor heater circuit (initially just the Upstream sensor). I replaced the upstream sensor but the problem persisted and in fact now both sensor heater circuits were showing bad. Her uncle took it to his "expert" mechanic who changed the sensor again and supposedly checked the wiring - after a couple weeks, and several hundred $$ in troubleshooting costs, he could not fix the problem and was talking about all kinds of things like replacing the MAF or the Cat, etc. Well, her Dad didn't want to sink any more money into it so I got the car back and started doing some more in-depth troubleshooting. What I discovered was that there was no power getting to the heater circuit at the female plug on the harness that the O2 sensor plugs in to. The fuse was good, so I started checking the wire harness closely. I found a spot on the underside of the harness where the harness had rubbed against the serpentine belt at some point and the belt had cut through the harness and literally cut the wire leading from the fuse box to the O2 sensor female plug (the sensor heater circuit). I soldered in a splice of wire and checked the continuity and it was all good!

FIXED! Or so I thought! I was still not getting 12v to the O2 sensor plug. The fuse was good, so what was going on? Well, I pulled the fuse and stuck a paper clip into the hot side of the fuse receptor and was getting 12v as I should! But, when I plugged in the fuse and measured at the top of the fuse (the little metal point on top the fuse where you can check voltage), there was 0 volts! Ahah! The problem was a worn female contact on the fuse box for the O2 sensor heater circuit fuse! Not wanting to replace the entire fuse box, I devised a jury rig to address the problem. I went to NAPA and bought some little "fuse taps" and shoved a fuse tap into the hot-side slot, then plugged in the fuse and there was now sufficient contact to provide 12v at the fuse and hence 12v at the O2 sensor plug! Reset the codes and viola - it's been working fine ever since.
 
Congrats on the victory. But it re-inforces my thoughts on trouble-shooting based on codes. Sometimes it's not the engine that has the problem - it's the diagnosis system!

diagnostics.jpg
 
Great work Bas! I love it when you can keep backtracking in order to find the cause of the problem. Most of the time they turn out to be simple but it’s only simple once you find the real problem.
Good stuff!!
 
Shops don't take the time to investigate these wiring problems because either, they don't know how or they know the customer is not going to pay for time to do it. Any car over 10 yrs old is not held to a book flat rate anymore due to age and corrosion of bolts.
 
LarryK said:
Shops don't take the time to investigate these wiring problems because either, they don't know how or they know the customer is not going to pay for time to do it. Any car over 10 yrs old is not held to a book flat rate anymore due to age and corrosion of bolts.
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Exactly, chasing down these faults is a PITA and at the regular rate of $60 an hour will cost a small fortune to find unless the mechanic is just lucky right out of the gate.
 
Good luck on finding a mechanic at $60/hour these days.
 
They are around, but remote and pretty much recommended word of mouth. Our average here for indy shops is $50-$75. Specilaized or really good indy shops are $160, same as dealers. I know of one in the next town at $45, but do not, repeat do not be in a hurry. My sign say $85 an hour, vintage British only, stops BS and wanta favors.
 
I do suppose it's been fifteen years or so since I was a mechanic, so my pricing may be a bit off.
 
Prices vary from area to area. I ince went to Burkesville, KY to fish at Dale Hollow Lake. Knew I had a bad u-joint in the van. Once we set camp we drove into town for supplies and I stopped at a one garage station with a bunch of gearheads sitting around. Asked if they could put in a u-joint while the wife and I shopped at the store a block down. Sure they said but part store closed, said I have the joint and they took over. Went shopping and bought an extra six pack, went back they were done. How much? $10. Easiest u-joint I ever replaced. Tipped them a six pack. Everytime we went into town they would wave and holler.
 
NutmegCT said:
Congrats on the victory. But it re-inforces my thoughts on trouble-shooting based on codes. Sometimes it's not the engine that has the problem - it's the diagnosis system!

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+1 ON THAT AND LOVE THAT DARTBOARD. The OBD systems seem to have gotten better after about 2010, IMO, the down side being that the tools have also become more expensive and model specific.

Tom
 
Agree on the specialized O2 tools especially. Just bought a set supposedly to handle all types. Old ones were not capable of reaching or too short. Just bought another scanner just for BMW/MINIs so I can reset more programs on my wifes 2011. Not cheap either.
 
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