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Discussion Starter #1
Since I did my cam swap (about 7 months) I've had these codes (heater circuit malfunction on both front O2s). These are the only codes stored in memory aside from a P1000 and the car runs and drives just as it should. I've done some searching but haven't come up with a lot of useful information related to just the front two circuits going bad.

All the fuses are good, and this issue persists across two different PCMs. If I remove either O2 sensor then turn the key on, the sensor gets hot after a short period of time - so I know they're coming on. The front O2s are Densos with only about 4,000 miles on them, which replaced the stock sensors after the codes showed up.

What exactly is the PCM not seeing that is causing it to throw these codes at me? Could there be a short to ground somewhere in the engine harness that is throwing off the return voltage? I'd like to get this issue kicked in the pants soon, because the car is due for emissions testing next Februrary and I would prefer to not have to work on the car next winter or take it to a shop.
 

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Mine was the sensor case being shorted to one of the contacts.

It was one of the back ones that never go bad, lol. :rolleyes:
 

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Discussion Starter #3
Normally I'd be all over that, but if that's the case why would the code show up all of a sudden right after my cam swap? Other than the cams themselves, the only thing I touched that could have had an effect on anything would be the harness (I had to unhook it by the a/c lines to move it out of the way to get the valve cover off). One of the things I have done is check for continuity to ground at the harness to the O2s with the key off, it checked out with next to no resistance. I was under the impression that the PCM completed the ground, which would mean I shouldn't have seen a complete circuit?
 

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I'm going to guess that something must have become fubared with the wiring. Taking off the valve covers and swapping the intake out moved around the harness near the back of the intake (which contains the wires to the oxygen sensors), so I'm going to guess that wires got broken. You'd be surprised (or maybe not) as to how fragile the wires get, especially back there where they get lots of heat. If you want, I can get you the wiring diagrams so that you can test for continuity between the pin at the oxygen sensor connectors and the pin at the bulkhead connectors for the heater circuits.
 

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Discussion Starter #5 (Edited)
I actually do have the EVTM, and I've been doing a bit of troubleshooting here and there as sanity permits. There's a good circuit between that connector by the firewall down to the connectors at the O2s themselves. I don't know if you remember but about two years ago my dad (llwta76) posted a thread about the car dying on me about a mile down the road, which was found to have been caused by a short in the wires to the left front O2 sensor. I checked those out and re-repaired the damaged wires just in case but no results. I also checked the signal voltage on all the sensors as the engine was warming up and everything looked normal as far as I could tell.

One thing that confuses me, however, is that when I try to measure for resistance for the heater circuits at the PCM connector I get continuity between all the pins? I must be doing something wrong.

What are the chances that the issue could be the PCM? I doubt that it's the issue based on the fact that the problem carries over across two (formerly) known good units, but the more I keep testing wires the weirder this thing becomes.
 

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What two pins are you measuring at? And at the actual PCM connector? You should be measuring between pins 71 or 97 (the red power circuit) and pins 93 (#1 heater return) and 94 (#2 heater return. If you have continuity between 93/94 and 71/97 with the sensor unplugged, you most definitely have a short in the harness. If that's the case, disconnected the bulkhead connector for the engine bay harness and test again. If continuity remains, the short is in the computer harness (not fun!).
 

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Discussion Starter #7
Actually when I did my testing at the computer harness I tried it with the engine harness connector hooked and unhooked. I tried between a bunch of random pins as well just for the heck of it, and I came across with every two pins that I tested being a complete circuit. I reasoned to myself that there couldn't possibly be a short on all circuits as I have no physical symptoms, so I assumed my problem was elsewhere. I'll have to check it out again to be sure I wasn't messing things up.

FWIW, just before I finished the cam swap after turning the crank by hand I cranked the engine over via the starter for a few seconds. One thing I didn't realize until later was that I forgot to hook up that harness before I started cranking. Could this have something to do with it?
 

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Nah, an unplugged harness wouldn't do anything bad. I would definitely test the harness again because as you pointed out the car wouldn't run if everything was one big circuit. Sometimes it's easy to get the orientation of the connector wrong and test the wrong pins, so check and double check.
 

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Discussion Starter #9
So, some measurements:

I measured resistance at the PCM connector with sensors plugged in and the bulkhead connector also plugged in. I saw ~6.8 Ohms between +12V and the voltage return for each heater. I also saw ~140 Ohms between the heater voltage return and ground. With the bulkhead connector unplugged I saw the same ~140 Ohms between the heater returns on sensors 3 and 4, but I saw infinite resistance on sensors 1 and 2.

Measuring at the bulkhead connector (male/engine side), I see the same open circuit measuring from the heater voltage returns to ground.
 

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Discussion Starter #11
The +12V (I should have clarified; the +12V circuit, not powered when I was measuring) was pin 97 on the PCM harness and the ground was at the plastic splice connector thingy (white plastic, several black wires going into it) near the PCM that bolts to the frame. The ground was directly at the battery when I measured at the bulkhead connector.
 

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You should be measuring at pins 3/4 (heater #1 and #2, respectively) and either pin 9 or 31 at the male bulkhead connector. The voltage returns won't have continuity with ground (either chassis or battery) because if they grounded there, the PCM wouldn't be able to measure.
 

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Discussion Starter #13
Right. For the bulkhead connector I was on pins 3/4 and 31, no continuity with ground. When I measured pins 93/94 at the PCM connector to ground I was getting 140 Ohms, but when I unplugged the bulkhead connector I saw no continuity. I did still see the 140 Ohms on pins 94 and 95 to ground though. When I measured continuity between the voltage returns (pins 93/94/95/96) I got a consistent 13 Ohms, this is with the bulkhead connector plugged in.
 

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Did you measure on the female side of the bulkhead connector as well? If you get continuity with ground there, then you have short in the computer harness.

And if you are wondering, the rear sensors aren't changing readings because they are fed through the transmission harness, which goes through the dash.
 

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Discussion Starter #15 (Edited)
Well, I tested the bulkhead connector with the PCM plugged in and there were no shorts to ground. I plugged the bulkhead connector back in, pulled the O2 harness and tested and sure enough, short to ground. So I decided to pull the rear section of the engine harness to see what was hiding under those wire looms, and this is what I found for the passenger side O2 sensor:



I'm betting that's a problem. :D
 

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Ham? 'Half Baked Mechanic?'
 

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Discussion Starter #19
what do you think happened for those wires to get like this? rats!
The wires being wrapped together like that in an insulated loom combined with heat and time, then finally my disturbing the harness when I did the cam swap.
 
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