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Alternate Causes P0135

12K views 6 replies 3 participants last post by  Marles 
#1 ·
Hi,

I have a 2002 base mustang, stock everything, 3.8L V6, manual trans. 2 years ago I had an engine light on for trouble code P0135. I replaced both front O2 sensors cleared the code and after completing a drive cycle the diagnostic tool said the sensors were fine. After about a month the code P0135 returned but I ignored it because I had already smogged the car and was lazy. Now it is smog time again. I checked the sensor's heater circuit and it read good 3.5 ohms, but I changed it anyway hoping that would be the end of it. That didn't help, checked the harness connector for voltage, ground, excess resistance, all checked out fine. Looked for vacuum leaks didn't find anything, all the fuses read good, the spark plugs look good. I went ahead and replaced the other three sensors on the off chance somehow everything stating P0135 is passenger side in front of the cat is wrong. That didn't solve the problem, but the car does run a lot smoother now.

Also when checking in monitor mode with the diagnostic tool I notice that the driver's side Bank 2 sensor 1 shows the heater circuit staying ON, but the passenger side (the problem) bank 1 sensor 1 toggles ON/OFF continuously. I need some suggestions for something that could be causing this trouble code. I appreciate any help you guys can give me.

Thanks,
Tiffany
 
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#2 ·
The P0135 calls out a circuit Malfunction. The term "Malfunction" means that something very basic to the operation of the circuit has gone wrong. Something sooooo wrong that the PCM is unable to further trouble shoot the issue.

Note the possible causes below. Some sort of wiring problem (water or connector) is called out. That is where I would start. Most likely in the O2 heater return ground.

If not able to find a wiring fault or dirty connector then the possibility of a bad PCM will need to be considered.


P0135 - HO2S Sensor Circuit Malfunction (HO2S-11)

Short to VPWR in harness or HO2S.
Water in harness connector.
Open VPWR circuit.
Open GND circuit.
Low battery voltage.
Corrosion or poor mating terminals and wiring
Damaged HO2S heater.
Damaged PCM.
 
#3 ·
All the wiring checks out good, I followed the trouble shooting procedure from alldata for this trouble code. I am at the point where the only thing that could be causing this is the PCM. Still I thought I would see if anyone had experience with this problem being caused by something else, something seemingly unrelated to the heater circuit before replacing a computer that might not need it.

Also can anyone tell me does their pre-cat bank 1 oxygen sensor 1 toggle on and off in monitor mode while the bank 2 sensor remains on? I am unsure whether that is normal or a symptom of the problem.

Thanks,
Tiffany
 
#4 ·
The O2s that are upstream(pre cat) are suppose to toggle from lean/rich to rich/lean pretty frequently. If they do not, typically that is finite proof that there is something wrong with that banks O2 or that bank has a problem with it. I would recommend throwing alldata away as it does short information when it comes to diagnosing.. its almost as good as the oreillys parts counter guys.

As wmburns mentioned, since he is quick with electrical responce, check the wiring. Reading the PCED(ford diagnostics) there are only 3 items that the pinpoint test leads to. PCM Wiring problem or HO2S. By chance what brand did you buy? Ive had bad luck with Boschs. I recommend getting a good brand of o2s and retesting. OE manufacturer of the o2s were NTK and i would not deviate far from them. Rock auto has a good price on them. If youd pm me id help you with the pinpoint test, guiding you through it as i cant easily provide it. I can also provide proper wiring diagrams if needed. Good luck keep us posted.
 
#5 ·
I should have clarified about the sensor toggle question. I was referring specifically to the heater circuit on the sensor switching rapidly between on and off repeatedly, not the voltage ranging up and down with trim adjustment. The voltages/trim seem normal, but the heater circuit on that one sensor switching on/off over and over seems like it can't be right. IDK

I used motorcraft branded NTK sensors that I ordered from rock auto. Thank you for the offer to help w/ the pin point test but I found the necessary diagrams and values at alldata, although it took much longer to find than it should. That site is so poorly setup to find the information you need it is ridiculous. I can see how if someone had a problem more complicated than mine it would probably be worth looking into factory manuals. I think at this point the only thing left to do is replace the PCM.

I will make sure to post how this shakes out. Nothing lamer than all the threads I have read through and the people never say what ended up happening.

Tiffany
 
#6 ·
I personally would start with one O2 for the sensor that is improperly toggling the heater circuit, my guess would be the one that stays on, due to the fact that pcms usually dont just "go out" after getting so many miles on it. Typically within warrenty times, 3/36, we catch all the faulty pcms and dont have a problem with the older ones.

I personally would double check the wiring from the pcm conncetor to the o2 connector and check for a short to power, with the circuit staying on that is the only thing i can see causing it with out looking up a wiring diagram. Good luck.
 
#7 ·
Well this thread is very old now but I thought I would come back and follow up on this problem. It was the PCM that was causing the problem, P0135. I was fairly certain that was the problem but not confident about replacing it myself since I would have to take it somewhere and pay to have it programmed, so I took it to a local repair shop. They checked everything again, wiring harness, sensor function, and decided it was the computer after a pinpoint test. I'm glad I had someone else replace the computer instead of doing it myself, they ended up having to do it twice as the first replacement PCM was defective.

So anyone else having this problem driving a completely stock, 2002 V6, it could be the PCM. Everyone, my father, my father-in-law, the dealership service manager, my husband, most of the internet, said it's probably not the PCM because they just don't break. I would say that is usually true, most people with this problem seem to find their harness is damaged somewhere. Just not this time.

Thanks to everyone who tried to help.

Tiffany
 
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