open loop fuel problem code 42

[hawgx2]

91 mazda navajo koeo codes are fine (11) koer code is 42 (o2 sensor) which I have replaced along with the fuel pump,fuel regulator, relay (all 3). I have checked all the fuses and every other sensor, and the injectors all seem to be ticking. The wire from the o2 sensor to the pcm has got good continuity but only reads .002 volts and does not switch . I believe the computer is running the 4.0 in an open fuel loop. I have also cleaned the mass sensor. I am going broke fixing things that are not broke can anyone help me PLEASE.
Thanks in advance

 

[MrShorty]

The key to your problem lies in the discrepancy between the 42 (O2 sensor idicates rich >0.5 V) and the fact that it is really indicating lean (<0.5 V). There has to be some sort of "communication error" between the computer and the O2 sensor.
1) Where are you measuring the O2 sensor voltage?
2) Is the PCM ground good? How about the O2 sensor signal ground? I'm not certain on a '91 whether it uses a 3 or 4 wire O2 sensor, but make sure the O2 sensor ground is good.
3) Make sure the O2 sensor is wired in correctly. If somehow the heater circuit were sending a constant 12V to the O2 signal pin at the PCM, you would have this problem.
4) It can be hard to diagnose faulty PCM's, but I once had this same problem, and, by process of elimination, the technician determined that the computer was faulty. Somewhere inside it was shorted out so that the computer saw a steady high voltage for the O2 sensor.

Good luck and let us know how it goes.

 

[hawgx2]

Thank you very much I will try all your suggestions
The o2 sensor is a four wire. I am checking the voltage at the o2 plug. Where is the pcm ground?

 

[Robb]

The 4-wire sensors have a black wire for the signal, a gray wire for ground, and 2 white wires for the heater.

Not sure if this helps you or not, I don't know about the PCM ground.

 

[MrShorty]

There's a black wire (I think it's black anyway) that runs from two pins on the 60 pin connector to the pigtail on the negative battery cable, I believe.

 

[hawgx2]

Ok thanks I will check that while I am switching out the pcm. Is there any certian percedure to folow for switching out the pcm or is it just disconnecting the battery?
Thanks

 

[hawgx2]

well I changed the pcm and the new one compensated for the problem better then the old one (not as much sut and oder from exhaust) but I still have a code 42 (o2 sensor) any one have any Ideas? Thanks in advance and I will post the solution when I figure out what it is.

 

[MrShorty]

What voltage do read for the O2 sensor if you read it at the PCM 60 pin connector? (Breakout box would be real nice for this, if you have access to one. Otherwise you can (gasp) carefully backprobe the connector.)
If you unplug the O2 sensor and run the KOER test, do you still get a 42, or do you get the 41 (lean) that one would expect with an unplugged O2 sensor?

 

[hawgx2]

voltage for the o2 sensor at the 60 pin connector is .002 I will try the koer test with the o2 sensor disconnected in a few minutes . again thanks for your help. Is it possible I got a bad o2 sensor from autozone?

 

[hawgx2]

I have got good voltage at the heater ground on the o2 and the o2 sensor has a good ground also a strong 12 volts to the sensor. I have .oo2 volts at the connector and at the pcm. I got a code 41 with the sensor disconnected
Thanks again I am open to any suggestions I am also thinking of changing the o2 sensor for the second time

 

[hawgx2]

I have replaced the sensor wire between the o2 and the pcm. Now i am getting .10volts instead of .oo2 should be .40 though Am going to splice above where i have previously probed by the connector and let you know what happens . I am still getting a code 42

 

[MrShorty]

You get a 42 with it plugged in and a 41 with it unplugged. This suggests to me that the problem lies on the O2 sensor side of the connector or in the connector itself.
I don't really know how common it is, but it is possible to get a bad O2 sensor right out of the box. I have seen a method for bench testing an O2 sensor using a propane torch. Set the sensor in a vise away from flammable stuff. Hook up a DVOM across the signal wires. Cold or in air, it should read 0 V. Put the torch flame on the sensor end of the O2 sensor and the voltage should go up near 1 V (after a few seconds to get the sensor element hot). I've done this before, and it seems to work as a means of testing the O2 sensor.
I've also seen described in the pinpoint tests (though I've never actually done it) that you can test the O2 sensor during the KOER test. Put your DVOM across the signal wires. At the start of the KEOR test, the PCM starts out running the engine lean then increases the mixture over the course of the test. By the end of the KOER test, you should have an output over 0.5 V to indicate rich.

 

[hawgx2]

ok I will try the tourch test. good idea. I spliced the new wire right into the o2 connector with a new pin and now I am back to .oo2 volts

 

[hawgx2]

changed 02 sensor still no go. now I am going to chang the spark plugs (pretty fouled up). Who knows at this point I'm just guessing.

 

[hawgx2]

well changed spark plugs and nothing still have a code 42 and runs rich. I am ALL out of ideas . Anyone got any?

 

[MrShorty]

Does the O2 sensor respond properly to the tests mentioned above?
While it appears that there is an electrical issue, can you tell whether it is actually running rich or not?
What does the fuel pressure look like?
Have you checked for shorts between the O2 heater circuit and the signal circuit?

 

[Glacier991]

I recall reading that the propane torch test is NOT a good ide for sensor health.... I cannot recall what or why, just passing on a recollection. (tried it once and it didn't work for me anyway)

 

[liqhx2]

i would recheck the FPR make sure it is not spilling fuel into the vacum also look for a vacum leak

 

[hawgx2]

Did all that, The signal wire reads .oo2 with Ignition on but with the engine running I get 1.0 volt at the computer from the signal wire. I have given up and taking it to a mechanic tommorow. Will let you know what happens

 

[MrShorty]

So that .002 V for the O2 sensor is with the key on engine off (KOEO)? That's normal. The O2 sensor can't generate a meaningful signal unless the engine's running.
The code 42 is reflecting that steady 1V from the O2 sensor.
One word of caution in taking it to a mechanic. Make sure they diagnose it correctly. I once had an O2 sensor code (a GM engine with the same code 42) that I decided to take to a mechanic. I replaced the O2 sensor before I took it in (like you have), and told him I had replaced the O2 sensor. Even so, about 30-45 min later, he calls me and says it needs an O2 sensor. I expect he had pulled the codes, saw the O2 sensor code, and decided that it needed an O2 sensor. I told him I would just come pick it up, because I didn't believe him. by the time I got back to the shop, he had pulled the car back into the garage and spent ~8 hours running diagnostics before determining that the computer itself was where the fault was. Not to say that all mechanics are lazy, or that it couldn't possibly be a bad O2 sensor. Just make sure that they diagnose it properly.