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Oxygen Sensor codes 135 141 155 96 Bronco

I read a earlyer post who had the same problem, He ended up fixing his bronco by replacing the PCM, Im hoping for some help with the trouble shooting, where is the PCM? could this be a problem with the PCM relay if there is one, oh and all my O2 sensors are newish replaced in the last year or so. I also have a low fuel pressure problem could this be the cause?

How does your Ford Bronco perform? What year is it, and size is the engine?
And no, symptoms, alone, are not enough information to make a diagnosis.

P0135 = Bank1 Sensor1 O2 Sensor Heater Circuit Malfunction
P0141 = Bank1 Sensor2 O2 Sensor Heater Circuit Malfunction
P0155 = Bank2 Sensor1 O2 Sensor Heater Circuit Malfunction

Taken literally, your O2 sensors have committed mass suicide. Not very likely unless you have killed the sensors somehow. I’ve never heard of anyone doing that, but I suppose that shooting a dose of something weird like Carbon Tetrachloride into the air cleaner could do it … maybe … perhaps …

OBD codes less than P0999 are predetermined by the EPA. But if and exactly how they are implemented is up to the manufacturer and is not – to my knowledge – data that is available to normal human beings. So we don’t know exactly how your vehicle computer determines that the O2 heater circuits have failed. Presumably either their voltages haven’t come into the normal operating range within the proper time for warm up or the O2 heaters aren’t drawing enough current or … who knows? Whatever made sense to the Ford engineers and the guy who programmed your vehicle computer.

The PCM is the one of the many names for a Car Computer (“Powertrain Control Module” maybe). They tend to be expensive.

As long as the vehicle thinks the O2 sensors are dead, it should probably run “open loop” which – in theory – means it will be driveable. It may pollute a bit more than it should, and gas mileage may suffer a bit. But you probably won’t destroy the engine.

I could be wrong about any or all of the above. But probably not.

I used to own a 95 Dodge Neon that turned on O2 sensor codes when it was feeling grumpy. Maybe twice a year. As near as I could tell, the codes had nothing to do with problems with the O2 sensors. I eventually found the computer fuse and pulled it for a few seconds when the computer complained about the O2 sensors. The codes would clear and not come back for months. Except when one of the sensors did fail in which case there was only one code and it came back after a couple of starts.

Where is your computer fuse? I haven’t a clue. If my experience is any indication, it will not be easy to find, and will probably not be labelled as a computer fuse. For the cars in our driveway, it is the radio fuse on two of them and I don’t know where it is on the other. You can also clear the computer memory by disconnecting the negative battery terminal for a few seconds.

The factory wiring diagrams for the engine control system of my 97 Explorer shows all O2 sensors spliced to the “HEGO system” fuse which is under the hood in the power distribution box. Nothing else is wired to this fuse.

Your 96 should have the same fuse. Have you checked it?

My Bronco runs fine, engine purrs, its a 5.0L V8 Fuel injected, EEC-V Management system.

This is not brain surgery, have you checked the fuse that I mentioned in my earlier post???

Take a look at the fuse list in your owners manual.

There should be a listing for the “hego system” fuse.

There is no fuse, it has a PMC relay, and a Diode in the power distribution box. Both appear to be fine as far as theres no corossion on the terminals and it doesn’t look damaged, is there any way to check these with a mutimeter?

If the PCM relay or diode was the problem your truck would’nt run at all.

The PCM relay sends voltage to the PCM, injectors, pump relay, MAF sensor, O2 sensor heaters etc etc.

SInce theres no fuse for the O2 sensors, there must be a break in the wire between the PCM relay & the splice to the O2 sensors.

If you’re interested in getting a VERY good set of wiring diagrams for your Bronco, check out the 96 Bronco Electrical & vacuum troubleshooting manual/EVTM
at For fixing electrical problems this manual is as good as it gets. And it’s only 21 bucks.

Something that you may not know. There is no rule that says that a DTC code, once set, has to be cleared by the computer in any particular hurry (or, indeed, at all) if it stops happening. Generally the computer will reset DTC codes, but sometimes only after the problem has not been seen for X starts or X “drive cycles”. The value of X (and the definition of a “drive cycle” for that matter) is at the whim of the engineers and programmer.

The problem causing the codes may long since have cleared – if it ever existed. The only way to find out is to clear the codes with a scanner/reader or by forcing a computer reset by removing power. Then see if the codes come back.

I ended up replacing the PCM and the codes went away. $50 bucks from a junk yard.