I have a '97 XLT, 5.0 and the error code for the O2 sensors has popped up, when checking it shows there isn't voltage for the heater circuits. I see there are 3 different heater circuits, 1ea for the left & right O2 sensors and a common one for the catalytic converter O2 sensors, so I ordered another PCM.
What do you guys think?

 

You ordered a new PCM ??????????????

The 4 02 sensors are powered by mini fuse 11 in the power distribution box which is under the hood on the drivers side near the firewall. Check the fuse.

 

Yes, fuse is good. I'm still getting up to speed reading the schematics but looks like the PCM provides the common ground path.

 

Which trouble code are you getting? If it's for a single 02 sensor the most likely problem is a bad heater in that 02 sensor.

Heres how the 02' heater circuits are wired

PCM power relay sends key on voltage to fuse 11.

Fuse 11 feeds a splice which feeds power to the 4 02 sensor heaters and each o2 heater is wired/grounded to it's own pin on the PCM

The diagrams you need are the 97 Explorer engine control diagrams.

Edit to mention that your 5.0 has 4 02 sensors. A 4.0 has 3.

 

To clarify slightly the above comments, the heaters for the oxygen sensors are grounded via the PCM. The PCM controls when the heaters are active, but an HO2S that is failing due the current draw of the heater is not likely to be due to a faulty PCM. You need to make sure voltage is making it to the heater element of the HO2S, that the heater element has appropriate resistance through it (not sure what that value is, but it should not be an open circuit), and determine if the PCM is providing a ground to complete the circuit for the heating element. And if the PCM is not providing a good ground, it still does not necessarily mean the PCM is the problem.

If you provide the exact diagnostic codes I should be able to provide the factory diagnostic steps to help troubleshoot the issue.

-Rod

 

Great help, much appreciated. I am learning as I go the OBD2, how to do the diagnostics and how to use the scanner. When I first read the trouble code it said bank 2 sensor 2 and I replaced it, it still reads trouble codes with it and now I see it shows all 4. It may have showed all 4 to begin with and I just didn't know how to read the scanner properly.
The connectors are hard to get to on the 5.0, situated high or above the transmission, so it is very difficult to access and get a good reading on them.

 

Like Rod said it would be good if you could post the code, if all else fails Autozone will read the code no charge.

If you want to check voltage, fuse 11 should have 12 volts with the key on & the light blue/orange stripe wire at each 02 sensor should have 12 volts.

 

Yep, I changed the O2 sensors on my 2000 Mountaineer with AWD and the 5.0L. I basically had to bear-hug the transmission to disconnect and reconnect one of the sensors. I wondered if there was an access panel on the transmission tunnel but never saw evidence of that.

The codes should have the format Pxxxx where xxxx is any sequence of numbers.

-Rod

 

The codes are P0135, sensor 1, Bank 1; P0141, sensor 2, Bank 1; P0155 sensor 1 Bank 2; P0161, Sensor 2 Bank 2.
One sensor has been replaced since the trouble code started, another replaced previously.

 

Well all the 02 heaters need 12 volts on LB/O & at the other end the ECU needs to provide the ground for that 12 volts.

If you are seeing the 12 volts at any of the sensors then it does look like the ECU is the problem.

My BIL owns an auto parts store and tells me that A1 cardone has an xlnt tech assist 800 number, might want to give them a call and if they agree the ECU is the problem you can order a reman ECU from them.

 

OOPS, so as not to confuse things ECU/PCM is the same thing.

 

Woof, glad you said that. I was thinking, 'daggone, got to order another part'. I've ordered another PCM and gave my VIN#, they aren't all that expensive. I really do love this little truck and want everything tiptop on it, I had hoped to have it repainted by now but this O2 thing is delaying the paint. I'm patient when the finished product is right.

 

Good luck with the project, I'll bet that is going to be one really nice 97 when you're done.

 

I'd really recommend checking the resistance from the PCM to the O2 sensors (or even just the ones you can access easily) to make sure the wiring is good before condemning yet another PCM. It's very unlikely that all 4 transistors within the PCM to ground the O2 heater elements are bad, slightly more likely that all five oxygen sensors are bad, including the new one that was recently replaced. It sure sounds like a wiring issue, something common to all 4 sensor locations.

-Rod

 

Will do and am prepared to do. One factor I haven't shared is that the truck belonged to the local water authority, a supervisors vehicle. It was rarely driven (less than 100K) but was driven on dirt roads. It was a mess, after I got the inside cleaned I took a pressure washer to the engine and didn't know where the PCM is situated. The engine was caked in dirt, I spent $8 pressure washing it and now have a nice, clean, tidy engine that don't run quite like it did when I got it.
It might be a 'self inflicted' problem.
I received notice that the PCM has been shipped, we'll know in a few days.