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My 2003 f150 with the 4.2 V6 engine has codes for no voltage from the heater circuit on both downstream 02 sensors. Upon further inspection, I found that someone who had the truck before me cut the downstream 02 sensors clear off at all four wires on both sides...and apparently did away with the harnesses leading up to them, because I can't find them to install new 02 sensors! Can anyone point me in the right direction as to locating the wire harnesses on the truck so I can replace the 02 sensors,or so I can at least get new harnesses? Thanks in advance, any input is appreciated.
Last edited by AndrewMTN; Mar 21, 2026 at 07:17 PM.
I have the wiring diagrams and can probably find it but before then, are your catalytic converters still intact? Kind of sounds like your cats got stolen and never fixed. If you don't have the cats the SES/CEL light will still be lit. Besides the spot for the sensors being gone if the second cats are gone.
Or maybe they got the converters replaced but not the wires and sensors. Just trying to understand the problem. Let me know and I'l post some pictures of the diagrams.
I have the wiring diagrams and can probably find it but before then, are your catalytic converters still intact? Kind of sounds like your cats got stolen and never fixed. If you don't have the cats the SES/CEL light will still be lit. Besides the spot for the sensors being gone if the second cats are gone.
Or maybe they got the converters replaced but not the wires and sensors. Just trying to understand the problem. Let me know and I'l post some pictures of the diagrams.
Hi BareBones, thank you for responding. I still have all the cats, the old 02 sensors are still there, someone just snipped the wires on them and I can't find where the plugs come off the main harness so I can replace them. No SES/CEL light, just two codes for each sensor (P0161 and P0141) and an off fuel mixture plus a rough idle.
The truck used to be a ranch truck, so it was subjected to some questionable stuff. It wouldn't surprise me if the guy who had it before me cut the wires because the sensors went bad or something like that. If that's the case, I just don't understand why he didn't simply unplug them if he felt the need to do so.
Last edited by AndrewMTN; Mar 21, 2026 at 10:03 PM.
Reason: Added context
I'll take a look but the downstream sensors only monitor converter efficiency. Won't have any effect on engine performance. Something to ponder.
If by "off fuel mixture" you mean fuel trims are high, that's a common problem with the 4.2. Usually it's hardened isolator bolt gormmets allowing a vacuum leak. You can buy the bolts with grommets for not much.
I'll take a look but the downstream sensors only monitor converter efficiency. Won't have any effect on engine performance. Something to ponder.
If by "off fuel mixture" you mean fuel trims are high, that's a common problem with the 4.2. Usually it's hardened isolator bolt gormmets allowing a vacuum leak. You can buy the bolts with grommets for not much.
Let me get in to my wiring book...
I see, in that case I'm glad it's not the downstream sensors causing it. Really appreciate the input! I'll definitely look into the isolator bolts
What kind of transmission do you have and is it 2WD or 4WD? Doesn't really matter a bunch because Ford strapped the O2 sensor harness to the transmissions. It connects to the PCM at the upper right fire wall of the truck, runs over the transmission and down to the converters and other switches/sensors.
Get your eyes on to the big connectors on the upper passenger side firewall and see if you can find cut wires or a bundle of wires. Maybe you're get lucky and it's there. It's a large square-shaped connector. Wires from other things are in the harness so there's no way the whole harness is gone. The O2 sensor wires are run in to the bundle with things like the speed sensor and reverse switch, etc. Edit - actually I'd guess they (one from each side) zip-tied on top of the transmission or something like that.
These pictures might help. C110 should be the connector.
Last edited by BareBonesXL; Mar 21, 2026 at 10:56 PM.
I think I would hit the junkyard and find the harness yours is missing
Right, it runs over the top of the trans and might be a stand alone harness you can easily replace
What kind of transmission do you have and is it 2WD or 4WD? Doesn't really matter a bunch because Ford strapped the O2 sensor harness to the transmissions. It connects to the PCM at the upper right fire wall of the truck, runs over the transmission and down to the converters and other switches/sensors.
Get your eyes on to the big connectors on the upper passenger side firewall and see if you can find cut wires or a bundle of wires. Maybe you're get lucky and it's there. It's a large square-shaped connector. Wires from other things are in the harness so there's no way the whole harness is gone. The O2 sensor wires are run in to the bundle with things like the speed sensor and reverse switch, etc. Edit - actually I'd guess they (one from each side) zip-tied on top of the transmission or something like that.
These pictures might help. C110 should be the connector.
It's a manual 4wd, I know the big connectors you're talking about, I'll post a pic of that later in case something isn't right with it that I can't see. Thanks a million for your help and the photos!
What kind of transmission do you have and is it 2WD or 4WD? Doesn't really matter a bunch because Ford strapped the O2 sensor harness to the transmissions. It connects to the PCM at the upper right fire wall of the truck, runs over the transmission and down to the converters and other switches/sensors.
Get your eyes on to the big connectors on the upper passenger side firewall and see if you can find cut wires or a bundle of wires. Maybe you're get lucky and it's there. It's a large square-shaped connector. Wires from other things are in the harness so there's no way the whole harness is gone. The O2 sensor wires are run in to the bundle with things like the speed sensor and reverse switch, etc. Edit - actually I'd guess they (one from each side) zip-tied on top of the transmission or something like that.
These pictures might help. C110 should be the connector.
So here's a photo of the big connectors. I did I vacuum test with a mityvac and it showed up low, it spent some time at 18 hg while rapidly vibrating but spent most of it's time at about 15 hg while vibrating, nothing was steady. Sure seems like might be heading towards new isolator bolts.
Here's something you can try to see if it is isolator bolts. The grommets around the bolts press down on the intake manifold, When they get hard they don't do that anymore and the gaskets leak. A person should be able to put their weight on the intake manifold and see a change if that is the cause.
But, you'll get a P0171 or P0174 code well before you get a big leak. The engine will run just fine with a small leak. You never said if you had codes. There are other leak sources also, like the PCV system.
If you have a scanner that does live data you can watch fuel trims also while you spray carb cleaner at the base of the intake manifold. It doesn't take much to cause a change in trims if there's a leak. Or you can press on the manifold and watch trims.
Looks like you're making progress. Is that electrical tape inside your harness in that picture? Somebody's been in there?
Here's something you can try to see if it is isolator bolts. The grommets around the bolts press down on the intake manifold, When they get hard they don't do that anymore and the gaskets leak. A person should be able to put their weight on the intake manifold and see a change if that is the cause.
But, you'll get a P0171 or P0174 code well before you get a big leak. The engine will run just fine with a small leak. You never said if you had codes. There are other leak sources also, like the PCV system.
If you have a scanner that does live data you can watch fuel trims also while you spray carb cleaner at the base of the intake manifold. It doesn't take much to cause a change in trims if there's a leak. Or you can press on the manifold and watch trims.
Looks like you're making progress. Is that electrical tape inside your harness in that picture? Somebody's been in there?
Yup that's electrical tape, there's electrical tape on the transmission harness near the speed sensor as well now that I've taken another look at it. No P0171 or P0174 codes, but there was one for the evap sensor, I'll have to look again to see what the code was. Perhaps I should do a fuel pressure test? The truck died once during idle today. I do have a scanner that reads live data, so I'll be sure to try that.
Last edited by AndrewMTN; Mar 22, 2026 at 05:59 PM.
Evap systems can cause running problems. What's the code?
You said that you think it's running lean. I assume that you assumed that? Live data will tell you. Watch fuel trims at idle and with revs.
It's code P0453 "high input from evap pressure sensor." It's not accompanied by any other codes related to the evap system, but I wonder if there's a way that it's related to the apparent vacuum leak. I took a spark plug off since my last reply and found it to be slightly wet with gasoline, so perhaps it's actually running rich or possibly trying to overcompensate for a lean condition?
Edit: I read the live data, bank 1 read "Open loop fault" briefly before switching to "closed loop fault" then switched back to Open loop. Bank 2 said N/A. Did a fuel pressure test just because and it passed.
Last edited by AndrewMTN; Mar 22, 2026 at 07:06 PM.
Your low intake vacuum might just mean that the cylinders aren't pulling very hard. Because the engine is running poorly. It doesn't necessarily mean a vacuum leak in the intake system. Live data fuel trims...look.
Here's a good description of your evap system code. Your engine looks pretty dirty and people have been in there. I'd check all of your electrical connectors and hoses. Some people will disconnect or plug things they don't understand. All of the "emmissions crap" is a common one. If the evap valve is open the engine will pull vapors from the tank. A vacuum "leak" that causes a rich mixture.
Your engine looks pretty dirty and people have been in there. I'd check all of your electrical connectors and hoses. Some people will disconnect or plug things they don't understand. All of the "emmissions crap" is a common one. If the EVAP valve is open the engine will pull vapors from the tank. A vacuum "leak" that causes a rich mixture. Here's a decent video. Not sure where your EVAP hoses are.
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