Hey, I have two 2005 FEHs, with ~150k miles each. One recently got a check-engine-light for emissions system; I plug in the OBD reader and pull up the codes p2195 and p2196. Quick google latter, and it seems I have to change an O2 sensor. Reset the codes, and drive the car to see which sensor is causing issues (upstream or downstream from the catalytic converter). Sensor data looks FINE!

I run it for a few minutes more, and then BOTH sensors drop off at the SAME time. So being in IT, I let the engine turn off then turn it on again with the defrost button. No change; no signal from either sensor.

But if I turn off the car and take out the key, open the door to let the computer turn off, and then restart the car with the key: I get appropriate signal from BOTH sensors. Until a few minutes later I lose signal from both again.

So, I'm thinking the ECU is knackered, but I'm hoping someone has any other suggestion for what might be the problem.

 

Well, swapped the ECUs between my two cars and the problem stayed in the same car. So not ECU. Maybe both signals get disabled when some sort of reset-able fuse goes off? I'll try swapping O2 sensors next weekend and see what happens, unless someone comes up with any better ideas?

 

o2's are like that. change em out and see what happens

 

If the o2s are original, you are at approximate end of life based on age or miles. Did both possibly go at same time? That would be unusual.

 

Last Update:
It's gotten to the point that I'll often turn on the car and there is no signal from either sensor, and I have to restart the car once or twice to start getting a signal, again from both sensors. In all this time, I've never seen one sensor fail independently.

I looked through the O2 sensor wiring diagram for the FEH (the manual I have is for 2006, but I doubt 2005 is much different in such a specific detail). Here are the parts involved:
1) The sensors themselves; one on the exhaust manifold, one after the downpipe/catalytic converter. Each has a heater circuit, and a sensing circuit.
2) The Powertrain Control Module (PCM), which controls power to the heating circuit in each O2 sensor, and receives a signal from the sensing circuit in each O2 sensor.
3) Fuse 7 in the Battery Junction Box, through which flows the combined current of the two heater circuits.
4) A body grounding point near one of the O2 sensors that provides ground for the sensing circuit in each O2 sensors and some other stuff as well (Manifold Absolute Pressure sensor, Fuel Rail Pressure/Temp sensor, Camshaft Position Sensor, Intake Manifold Runner Control Monitor).

Because I have two of these cars, I tried:
1) Swapping the PCM. All I got was some ABS complaints in both cars, but no change in which car was having issues (apparently might be differences in the PCM/ABS units within the same model year. Didn't check the ABS unit, but the PCM parts number was different by 1 letter. The power-steering computer in one car was replaced with a newer one in the past, so maybe that might be involved as well.). Put the PCMs back where they originally were, and all the ABS errors went away.
2) Checking Fuse 7 for continuity. All good.
3) Swapping the top O2 sensor, and again the problem stayed in the same car.
4) Inspected the ground-point and it's cable. Seemed to be in good condition, but I unfastened it, cleaned a bit with a wire-brush and re-fastened it. No change. It was a long-shot anyway because with the number of sensors that are grounded by it, if it was the problem the car would have a lot more complaints.

And that's where I left it. I didn't get to swap the lower O2 sensor (I ran out of time/wife ran out of patience). If I had to guess, the culprit is likely an intermittent fault somewhere in the engine wiring harness, the grounding line, or possibly the lower O2 sensor, that when it short-circuits or whatever induces the PCM to turn off both sensors until next time it gets rebooted.

The car giving all this trouble was my wife's daily driver for 5 years with very few issues, but towards the end of last year I had to replace the throttle body, the exhaust flex pipe, and the MECS pump. She had been talking about getting a newer/smaller car for a few months, and after spending all this time unable to fix the current issue I finally agreed with her. So we ended up trading it in for a few dollars off a CPO subcompact CUV.

Luckily my FEH is still going strong, and hopefully will keep going until I can afford a CPO Ford Escape PHEV

 

I assume you were clearing OBDII codes as you went to see if they instantly reappeared? By not clearing the codes, you may have fixed the issue but the DTC would remain on for perhaps several drive cycles. Just asking for clarity for the board.

 

No problem with asking for clarification; the reason I wrote such a long follow-up was for the benefit of the board, or anyone else with a similar issue that found the thread.

Yes, I cleared all codes every time they showed up. However, the main way I was diagnosing the issue wasn't with the CIL and DTC code. It was with a bluetooth OBD II reader and Torque on my cell phone: With that I was able to graph the signals from both O2 sensors, as well as see if the engine was running closed-loop (using the O2 sensors to calibrate combustion) or open-loop (ignoring the O2 sensors).

These images aren't mine, but they show what O2 sensor data should look like:

Sensor 1 is pre-catalytic converter, Sensor 2 is post-catalytic converter.