P0155 code on 96 F150

I have a 96 F150 w/300ci 6,5speed-4x4. It has 60,000 on it and I have replaced the bank 1 sensor 1 o2 sensor 3 times now, each time it has fixed the 155 code(heater malfuction) but this time no such luck. I cleaned and blew out all connetions to the computer and still no luck. Last time it did this I replaced the sensor and then I got codes from the other sensors with heater malfuction, I sprayed them with W/D 40 and finally got everyting back to normal, this time I just got more o2 codes. Does anyone have an idea on what to check for before I take it to a dealer??

I'd be looking for a wiring problem in that circuit at this point. Unless something is causing the 02's to fail

Thanks, that was my thought, just have to figure a way to do this without undoing the wire covers to see wire colors

how do you plan to locate the wire probelm without opening the harness up?

I have the EVTM manual and I used a meter to check for continuity and everything checked out to the PCM, tried switching O2 sensors from #1 to #2 and still have the P0155 code, I have to think it is in the PCM not recognizing the heater on #2 sensor, any other thoughts??

continuity only confirms that 1 strand of a multiple strand wire is good!
will the circuit push the current required through it? any fuses blown that say 02 sensor? so you think it's the pcm then ?

That could be true, but the meter would have shown that as a low reading I think. I checked the fuse and even replaced the one for the O2 sensor heater, switched around the circuit breakers and checked all of the rest of the fuses under the hood and found nothing. I don't know if it is the pcm but I believe if I don't find the problem I am going to have to take it to a dealer, they have far more test equipment then I have. Thanks for your ideas though, I appreciate it--2 heads better than 1

you are using a digital volt ohm meter right? I've seen more people tricked by the method you're using then I can remeber, especially when you can't verify the integrity of the wire, connectors etc, visually!

I used both an analog and a digital meter and they both confirmed continuity.even if it were only 1 strand wouldn't current still pass through to the pcm, as it is only sending 5 volts. I guess what I am going to do is go to the dealer because I don't feel comfortable tearing into a wireharness with 90 some wires in it, unless there is another way of checking them out.

To prove my point try to power up an old sealled beam headlamp with a couple of strands of wire out of say a 16 gauge wire, on both the power and ground side. Same idea as trying to boost a car with a tiny wire or really light gauge booster cables, the voltage will still go through but the current won't! Your IAC motor is an electric motor and probably draws at least 5 amps when it's moving. Some 02 sensor heater circuits draw 20 amps so thinking that continuity is good enough is like I said an error alot of people make. Voltage isn't the problem! It's the current required to heat up the heater elements. Maybe it is a PCM but they aren't a common failure from my expereince.

good luck and keep us postted

Jeff

Well I understand what you are saying, which gives me an idea of checking the voltage going to the O2 sensor rather than thinking it is the signal going out to the pcm. I will check the amps coming into the sensor, does that sound good?? Or is there some other way like taking 12 volt right to the sensor to make sure I can rule that out. I don't know if I told you I had this problem a couple of years ago and I replaced the sensor in question and then it went to another sensor then all three. I chased it around for a few days' so I decided it was a bad connection and sprayed all 3 with WD-40 and no C/E light till a week ago. Tried the WD trick again, this time I lost everything not only heater but funtion of the sensor itself, needless to say it was running very rich, so I dried everything out and it all was gone except the P0155, thats when I changed sensor. I have checked for voltage at the sensor but never amps so I will try that next. Thanks

if the dvom you are using has only a 10 amp fuse in it and most do and the heater circuits draw more then 10amps you'll blow the fuse, You also need to verify what the proper voltage is for the heater circuit, 5, volts 8 volts or 12, some are different. With a scanner with bi-directional controls you can actuate the heater relay and check at the sensors for this voltage. Otherwise you need to check it when the truck is cold and running. If all sensors are dead then you've got a wiring issue or pcm driver etc, if only 1 then it may just require the sensor or the circuit to that sensor repaired!

Like Jeff said just because you have continuity does not mean the circuit is still capable of handling a load. Example another tech in my shop replaced two horns on a truck because they did not work. After replacing them they still did not work. He spent a couple of days trying to figure it out then the service writers gave me the truck. If the prior tech had also done a voltage drop test he would have found that a splice in the circuit right before horns was bad. I fixed the splice and horns started working ok. Like I said this is just an example of what Jeff is telling you.

Thanks for all the info Jeff, it is getting cold here so it is time to head south for the winter. I think when I return if the truck hasen't healed itself somehow I will take it to to Ford dealer and let them have a crack at it, I just don't feel confident enough to tear into that harness. Thanks again for your time and info on the subject.

I 'll keep my fingers croosed but doubt it will fix itself!