Not sure what 87 XLT was talking about. The wiring diagram on autozone shows no relationship between O2's and transmission except that they are both independently wired to the PCM. Where'd you get your diagram from, Chiltons or another book?

Your engine shutter on shift is likely due to a poor running engine rather than the tranny itself.

I have one question on your codes because my code book says 156 is Bank2, #2 is bad sensor circuit. Code 161 says bank2 # 2 bad heater circuit. All others match what I have.

Yeah though the wring diagram shows the heater side of the O2's to be coming from fuse #11. Nothing else is powered by that fuse. And that fuse is the only thing common to all sensors' heater circuit. Each heater grounds out to the PCM - I guess so that by a resistance check, the PCM knows the O2 is warmed up.

You can try this test, remove all the O2 connectors and volt test each one with the key on. Should get 12 volts at the light blue/orange striped wire at all 4 connectors. Voltmeter touching connector and then other to ground. If you get nothing, you have a direct short upstream of the sensors. This would address the heater circuit of the O2's.

For the switch side of the O2's, there is a common wire feeding all of them. And then each one independently feeds the PCM on the otherside of the switch itself. There is a gray with red stripe wire feeding onto a solid orange wire at all 4 sensors. The schematic is goofy here because it says the gray with red wire is feed from the Cam position sensor. The Cam psoition sensor is feed from the PCM. Now this whole side of the cicuit throws me because my understanding if an O2 sensors is that it generates a voltage once heated up and sampling exhaust gas - kind of like a thermocouple that generates electric voltage from heat and dissimilar metals on the heat source.

On my 93, the circuit goes from ground to O2 to PCM where the PCM measures the vlotage. I don't know why the O2's in your case would ground to the Cam sensor. But I guess you can check the orange wire on the connector for open with the key on. If they are all open, you have no ground path for the O2 and thus get a 0000 reading like your mechanic said.

Hum - this makes me think you migh have a bad Cam position sensor or a bad connection there. You can alway bypass this, I think. Could try a test - connect the orange wire from a couple sensors to ground, disconnect the pin from the cam position side of the connector. Clear all you codes and warm the car up - see if those 2 O2's throw thecode or not. If not, and the other two still do, there's some thing going on with the ground to the cam sensor.

This is really goofy but from what the schematic says, this is what I would try.

Only other thing I can think of, because your schematic is so different from mine, is that you may have been given the wrong type of sensor. I have also found Bosch to last much longer than Eilchen (not sure of spelling) ones. Autozone lists them as the same though.

 

Thinking about it, I've seen other errors on Autozone's schematics. Mine has a missing ground on one of the O2's. I would first test for a connection from on orange wire at a O2 to the gray/red wire at the cam sensor connector. Do an Ohm check for continuity. If it's open loop, the schematic is wrong.

 

The wiring diagram I'm looking at is the Ford EVTM for a 97 SOHC.

It shows pin 91 of the PCM wired to a splice which is wired to the O-2's (one gy/r & 2 solid orange) & trans controls. Yet another reason that makes me glad I dont drive automatics.

BTW the OHV wiring is different.

As you say the PCM provides the ground for the sensor heaters. I wonder if there could be a common ground problem within the PCM?

 

Yeah I saw the 2 different diagrams on autozone and pick the OHC one. The OHV one only had 3 O2's. So I was looking at what Autozone thought was right. You are seeing a different schem than I - look at the one on line. Either way, you can always do an ohm check to see what is connected to what. the first test I mentioned will make sure that at least you are getting power to the heater portion.

Wait - you say PCM spliced to 1 gry/red and 2 org - where's the 4th O2?

I don't see a PCM internal issue here. If there are 4 heater grounds to the PCM then the program wants to see 4 different inputs - thus they are all different circuits. I think the program looks for a fixed resistance which means it's all heated up now - does this for each O2. Besides - you replaced the PCM right - the last thing I always would recommend by the way.

 

The switched side of 2 O-2 sensors are spliced to the GY/R, which accounts for the missing O-2.

Since you say that the 4 ground pins of the PCM are independent circuits, I guess my common ground theory is out the window.

BTW the EVTM doesnt show any connection between the O2's & cam sensor. Gotta love this electrical stuff.

 

OOPS, i took another look and the cam sensor is also spliced to pin 91 along with the trans controls & O-2's.

What i'm looking at is the "engine controls" diagram in the EVTM.

 

What is EVTM - Ford's own repair manual?

 

Yeah, it's the Ford "electrical and vacuum troubleshooting manual" For fixing electrical problems it's the best manual to have. It has seperate wiring diagrams for ALL the different circuits.

Much better & easier to understand than the wiring diagrams in Haynes or Chiltons.

BTW, the only thing that pin 91 does is to supply voltage to the switched side of the O-2's, the cam sensor & trans controls.

 

So Demcdona - what's the latest?

 

Hey guys,

I was out of town this weekend...I needed to get away from this car issue to clear my head and try to get started on a new course of action this week. First let me thank you guys for your concerted efforts to brainstorm this problem, I'm truly fascinated by your ability to bounce ideas off one another. Here's where I am at now:

I actually took my Explorer to the Ford dealer down the highway from me and paid the $90 to have it checked out and the mechanic there told me I needed a new PCM for $650, and that it would be a special order and that would cost extra. I went back to my mechanic and was infrormed that while my truck was in the shop over the weekend they tried that with a PCM from the exact same model of Explorer they had in the shop and that it didn't fix the problem. So I went back to the Ford dealer, told them what the deal was, and they looked at my car again. After two hours he concluded again that since all four O2 sensors were out, that it had to be the PCM. I told him thanks for his time, and went back to the mechanic I have had work on my car armed with that information.

My mechanic ordered up a new PCM, it came in the next day, and on Wednesday he installed it. Still the same problem. That's where I am now.

I just got back from work to start going over what you guys have suggested my next course of action should be, and I have to say that I'm a little confused. Most of the tests you suggest I won't be able to run myself because I don't have the tools or the shopspace to work in, so should I just pass this on to the mechanic I've had work on my car? Wouldn't this be something that a mechanic at a Ford dealer would have access to? Because from what I saw, they didn't run any of these tests. Also, are the wires coming from the O2 sensors as difficult to access as the sensors themselves?

In terms of O2 sensors being the correct ones for my truck, I called the dealership to get the part numbers for the ones I needed, then picked them up wholesale from a part supply place in town. I paid a little extra for the Motorcraft ones as well.

At any rate, would it just be best to go back to the Ford dealer to get this resolved, or should I head back to my mechanic armed with your advice?

Thanks again for everyone's time, hopefully this problem is working toward resolution and that Jharger can continue his streak of correct solutions. I really appreciate the assistance, and I look forward to your next responses

Best Wishes

 

The confusing thing to me right now is the 2 or 3 different schematics we have seemed to find. Of those, only the power from fuse 11 which runs the heater circuits inside the O2's is the same. So If you are getting O2 not heating up codes on all 4, I'd say the O2's are note getting power from fuse 11. You checked the fuse and it's OK. So I suggested checking for 12 V down at each connector.

Then, if you are getting power, the other end of this heater circuit grounds inside the PCM with 4 different inputs. The PCM is a computer with I/O's just like the one we are talking over. it has a keyboard and a mouse input - 2 different ones for 2 different things. So why would Ford have 4 different O2 inputs if they were all connected together on the inside? Doesn't make sence. So there is no correlation for "if all are bad it must be the PCM". This is a typical response form a mechanic that knows nothing about computers and if they do, then they didn't take the time to study the schematic to see how the computer is supposed to work.

I didn't suggest PCM because the heater circuits are all independent once they hit the PCM. Think about it, with 4 different inputs, Ford can then tell you which of the 4 may ever be bad, independently, and that's why there are 4 different code numbers. Otherwise, you would have just one that said "O2 system failure" and you would then have to go find out which sensor(s) was bad.

it is not common for all 4 O2's to be bad at once so it is right to look for commonalities, i.e. fuse 11 power. Now I still have that question about the codes as when I looked up the numbers you gave, 3 were for bad heater and 1 was for a bad sensor - the sensor O2 feedback side (2 sides to the switch, thus 4 wires/switch.) it would be good to verify what codes you are getting now just so we are sure about what we are looking for.

Yeah sure, take all his stuff down to your mechanic. If he diagnosed the PCM and it didn't fix it, you don't pay for that. But tell him to show you the schematic and how he intends to troubleshoot. If you have the ground side of the O2 going to either the cam position sensor or the transmission controls, I'm at a loss there. Neither of those scenarios makes sensr to me since a O2 sensor generates it's own output voltage. The PCM inputs read this and the other end should just go straight to ground.

 

I've reread this post, and keep coming back to the #11 fuse terminal. If there's indeed power going to the fuse, as Demcdona suggests, then the next step would be to check for continuity between the #11 fuse block (output side) and the first power splice to the sensor heaters. I think there's an open either at the fuse terminal, or at the multi-heater splice......Maybe a simple solution to a complicated appearing problem.....?

 

That's what 87 XLT and I think too. But ranger - what about the other side of the switch circuit? The 4 leads into the PCM are clear but the other side should go to a clean ground, I thought. The various schematics reviewed sufggest it goes either to the cam sensor or transmission controls.

 

Hey guys,

I went back to AutoZone today to recheck the CEL codes my Explorer was putting out, and they are as follows:

P0135 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P0155 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
P0156 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)

The codes are the same as they were initially. My mechanic first said that it wasn't the PCM (he checked that himself with a PCM from the exact same car), it was the Ford mechanic who said the PCM needed replacement (seemed like a default answer on behalf of the dealership to me, but hey, what do I know?). Since my mechanic said he couldn't fix the problem, he said I should take it to the dealership since they would be more experienced dealing with the problem, and that they would be able to tell me more precisely what the problem was, and know how to fix it.

The first day I took my truck to the Ford dealer I just waited in the waiting area while they checked out my problem instead of asking to watch them troubleshoot. That was probably a mistake on my part, and when I brought my truck in the next day to explain that the PCM wasn't the problem, I demanded to oversee what troubleshooting tests they were running. From my recollection, they didn't check any of the wiring, pull out a wiring diagram to inspect how things work, or even check whether there was power at any of the four O2 sensors. Both mechanics that worked on my truck at the dealership seemed perplexed, and once again, they defaulted to "replace the PCM...$650.00". So that is why I went back to my mechanic to have him replace the PCM...because the mechanics at the Ford dealership must be right.

So after reading what you guys have mentioned, I should have my mechanic (or go back to the dealership and tell them they were wrong with their diagnostic and see if they will again re-inspect the problem...can I even do that?) check to see if there is an open either at the fuse terminal, or at the multi-heater splice? And also, check to see if fuse #11 in the power distribution box is working (even though I checked it out and it was fine, a second, more experienced look couldn't hurt), and then if that checks out, proceed to inspect whether or not there is 12V down at each individual sensor? And to top that off, get them to go over a wiring diagram to locate the potential failure points common to all four sensors?

Whew, that was a lot, but once again, I am constantly learning here, and I am incredibly impressed by not only your collective knowledge, but also your dedication to helping a total stranger fix a nagging problem. Thanks once again guys, I really appreciate your help and suggestions!

Best

 

Uh, what was that again, I didn't quite hear it.... Seriously, CMan, 87, and Jharger have done the grunt work on this one.....

Demcdona, was this tool a fuse tester or a circuit tester? If it was simply a fuse tester, we still don't know if there's power to the #11 fuse, since it only tests a good/bad fuse.

Since there was no MIL showing until about a week after the O2 sensor replacements, I think we're missing something here. Apparently the new sensors did do something to the heater circuits to make them work again, at least for awhile.....?

One other thing, did the MIL light up while driving, or was it on just after starting the Explorer one morning?

ROE