2001 Jeep Wrangler - 4.0L Code PO132

[Ruger77]

We have a 2001 Jeep Wrangler 4.0 that came in for a driveability issue / check engine light on. The codes were, and are, PO132 and PO138, which are both upstream and downstream O2 sensors shorted to voltage. When I monitor datastream, both O2's read 4.98 volts and 4.76 volts with KOEO, and after you start it, the upstream voltage will start dropping very, very slowly if you rev the engine to 2500 rpm. Eventually, (after about 5 min's) you can get the upstream O2 down to .5V and sort of toggling, but if you let the engine idle, the voltage starts creeping back up until it is back at 4.5 volts. The engine will actually run pretty good until that point, then, if you have cleared the codes, and the computer goes in to closed loop, it really starts trying to lean it out and will eventually stall. If it kicks the code before it stalls, it will go into open loop and run a lot better. I have over 4.5 volts on the scan tool for both O2 sensors, if they are plugged in or NOT. We have put new OE O2 sensors in it, along with a PCM rebuild, and trying a junkyard PCM. I have dissected the connector at the PCM looking for possible touching connections and found none. All the same results. According to the Mitchell flow chart for this code, if the wiring does not have a short to voltage, (which it doesn't), and the voltage is over 1.5V with the O2 unplugged, then the PCM is bad. We have checked the PCM grounds and the O2 grounds, they are good. I even scoped the grounds looking for a bad connection after the circuit was loaded, but no luck. I have a feeling I'm not seeing the forest for the trees, but maybe someone can help? FYI, the engine does not act like it is running rich at all, and actually runs well if it isn't trying to listen to the O2 sensors and lean it out.
Thanks for any help,
Scott

[steven kiser]

let me ask this. what happened before the codes popped up. was it at another shop for work, did they just fill the tank? this almost sounds like a fuel issue. i've seen issues like this with the alcohol added fuel.

[Tim Martin]

What is the KOEO voltage for both of them? As I understand the code, they are electrical codes. Somewhat points to a faulty PCM but do not circumvent proper diagnosis before replacing it.

[ricmorin]

What you describe sounds like the heater circuit for those O2 sensors is open or has high resistance. The PCM supplies 5v to these O2's and seeing 5 v cold is normal. After about 2 minutes of running it should toggle IF the heaters are working. Check your heater circuits. Mouse chew-throughs and rub-throughs are common causes of this.

[Tim Martin]

Don't they have separate codes for the heater failure though?

[ricmorin]

Yes, PO135 and 136. But a high resistance might prevent that code from setting. Plus in 2001 the stategies were not that complex as they are now. I'm not saying it is the heater circuit, but the conditions match exactly.

[Tim Martin]

Well yes, I can agree with what you are saying. We all have seen strange things in these electronic devices. It seems kind of strange that you would have the same type of code for both sensors though.

[ricmorin]

On the contrary, I would suspect the heater circuit because it's the same supply and ground for all sensors. What would be stranger is 2 sensors being faulty or two separate circuits shorted. It can happen, but less likely?

[Tim Martin]

Well, see, I was thinking that if it were a heater circuit fault, it would set a heater code. Not?

[fortknoxx]

look for signs this jeep was prev used as a submarine or a duck boat. many of the kids around here have amphibious jeeps. we look for water lines (marks) under hood and dash. mud under carpet and mats. once it is confirmed to have been swamped all bets are off. let the insurance adjuster total it out. see my customers 2000 Jeep Cherokee. I have to admit the sunset is beautiful.