OK, got a P1131 code. I've looked everywhere for vacuum leaks, and can't seem to find any. Here's some additional info:

I'm able to monitor the PIDs while running for the O2 sensor voltage. I'm noticing that the voltage for sensor 11 and 12 are both near zero and don't move much. The voltage for sensors 21 and 22 are both around 0.5 volts and bounce around a lot as I rev the engine. Generally I would conclude that I've got something wrong with the passenger side (bank 1) here, but it seems crazy unlikely that I'd have TWO bad O2 sensors. I've also tried disconnecting a spark plug while running to see if that causes the O2 value to spike. Doesn't seem to.

Some additional info: -----------

The O2 heater currents are about 1A for the forward sensors and 0.7A for the rear sensors.

The LONGFT1 and LONGFT2 (long term fuel trim bank 1 & 2) seem to both be normal. The MAF voltage seems to be a bit higher than the normal ranges, but only about 5% higher than normal.

Barometric pressure PID is reading about 88 kPa even though I'm at about 2900 ft so that's about 1000 feet off. That seems off, but not too terribly off...

Almost forgot, it's a 2000 4.6L

 

If your sure the trim tables are near normal without feedback from the sensors on that bank, there maybe a harness issue.
Normaly with no OX sensor feedback the fuel table goes rich for engine protection.
If the sensor cannot function, taking a plug wire off on that bank can't be seen by the Ox sensor, either.
Harness cross to another lead or a cross to ground on the sensor section.
Normally the rear sensor does not move much.
With both being near zero, look for a harness issue.
Neither signal is getting back to the PCM to be seen by your scanner.
Good luck.

 

Either replace the O2S11 or swap the two upstream sensors and see if you then get P1151 DTC to confirm the sensor is bad.

Be certain that the PCM's KAM is reset if swapping sensors. Unhooking the battery while working on the sensors will do it.

P1131/P1151 DTCs are fairly reliable as to detecting a faulty sensor.

 

Thanks guys. I've confirmed that it is indeed a harness issue. I followed the diagnostic procedures in the service manual and found an intermittent short and an open. Seems the pack rats have been busy!

So, now I'm a bit stumped. The harness going to that sensor runs up the back of the engine. How on earth do I dig into that and replace the offending wires without removing the whole engine?

 

Getting in from the wheel well area may locate the connector.
The harness is a seperate item with a part number on it should you choose to replace it.
The harness feeds a lot if items. The ox sensors, the transmission etc.
The fuse feeding the whole deal I believe is #23.
If that fuse blows from a short you get all kind of effects and get a long list of codes.
Last known cost for this harness was about $110.
Good you found it.
Good luck with it before I get banned from this site for defending my positon in another thread.

 

Thanks a lot. Any idea what the gauge of the wires are in those harnesses. It seems that 95% are all the same. I'm thinking it's either 18 or 16, but I can't tell for sure. I've got to make sure I pickup the right size splices at the store.

 

I'm not sure of the wire gauge but the splices should accomodates both sizes.

 

Ok, ran 4 new wires and spliced in at the big honking connector at the top of the engine compartment. It all works now. Wire gauge was 20, but I only had 18 gauge wire.
I think I can guess why the fuel rail PIDs were normal even though the O2 sensor was at 0. My guess is that once it senses that there's a problem with the o2 sensor, rather than dump a ton of gas into that bank, it uses the other side as a proxy. That's why I saw both sides at exactly the same value. Pretty smart those Ford guys.

 

Good work, good luck.