HELP! Troubleshooting guru needed: P0155 will not go away

Hoping someone here knows where I can begin to track down this problem as Googling and forum searching has not revealed any answers.

2005 Mustang GT, K&N cold air, matching Diablosport K&N FIPK tune (both in the car for about three years now, no issues)
Problem: Constant p0155 code

What I have done so far:
>Replaced the B2 S1 sensor with three different new sensors (one OEM Ford purchased from eBay, two NTK from PepBoys). Cleared the code, each time after swapping sensors, but it returns within minutes of running or driving.
>I have pulled the FIPK tune and reinstalled the stock tune, same results. I reloaded the FIPK tune, still the same results.
> Checked for corrosion and burns on the harness; nothing obvious


I'm pretty sure there is something other than bad sensors going on here; does anyone have any ideas? Could really use some sage advice.

When u swap sensors did u just swap b2 s1? Try swapping it with a sensor u know is gd like b1s2 and see if the location changes.


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+1, that's always an easy way to test for a bad (or wrong) sensor!

Also, which side DID you actually change, passenger or driver side? Bank 2 Sensor 2 is driver side front sensor.

Lastly, it is a heater circuit malfunction, so you could have one of the wires pinched/cut somewhere between the PCM and the O2 sensor. A simple voltage check at the +12 heater pin on the o2 sensor would tell you if voltage is missing or present. (but, since you're only getting 1 code, this is probably not your issue).

Probably not it, but check fuse 47. Normally when fuse is bad you get multiple codes for O2's, cam position sensors, and the EVAP system because all of these sensors are on the same circuit. Short it out, fuse blows and you get codes for everything. What are you using to pull code P0155? Maybe your DTC reader isn't pulling everything? My SCT X3 wasn't pulling any codes even though my check engine light was on. Borrowed a friends code reader and it pulled everything. My SCT probably needed a firmware update or something.

Hey guys, still the same, possiblly getting worse

I did check fuse #47 (as well as all other underhood fuses) and it's fine.

MAF is not dirty

Harness shows no visible burns or damage

Used an Innova handheld to pull codes and run live data. Still gets p0155 instantly after clearing, but no other codes. Live data shows the sensor is switching voltage, looks the same as the other bank

Car struggles to crank if it has been sitting for a while. Literally have to feather it like an out of tune carb to get it to idle. Idle is still low (~650 rpm) and really weak.

Engine runs smooth, but is weak and doesn't sound quite right.

Mileage is like single digits around town, and still low at highway speeds

I feel like something had fried that I can't find. Before all this started, I ran the car up to like 120 on a wide open section of road (little testing for upcoming track days) and right before I backed off I caught a quick sulphur-ish electronic burn type smell. The check engine light with the p0155 code came on shortly thereafter. I assumed I had just killed an O2 sensor

Have you checked the plugs and COPs?

The sensor can still switch if the heated circuit is bad I believe. For it to instantly come on sounds like a short or open. Check behind the intake where the wire harness goes over the trans bell housing. This area sees a lot of heat and breaks the wire conduit down and leaves the wires exposed. I've seen a few cars with this and the wires rub on the trans and short.

If you've change sensors with a known good sensor it has to be wiring. You can check the harness I mentioned or start ohm checking the wires.

I did confirm fuel and fire by pulling the COPs & injectors one at a time while the car was running

That was my thought, but I don't know what it where the issue could be. So far the wiring looks totally fine. I will be tracing it more this weekend to see if something is hidden

Bump, because I still need help and trying to avoid a major shop expense
So far all indications are pointing to the PCM, but there is still only one code and all systems functioning, and car is technically drivable, but it's very weak and running rich.
Does that sound like a possible scenario to anyone?

I had a similar issue using universal o2 sensors. I would ohm check the wires first. Did you check the car side harness to make sure none of the plastic melted and the male side of the o2 sensors are seating properly? Or maybe a wire stretched and broke inside of the sheathing.

While a tech for ford for 12 years pcm where always a last result. In my younger years at ford a few pcm got installed that didn't fix the problem. After that I always ohm circuit wires from the pcm to suspect connector ends. Even then once I thought I found a problem would load test that circuit.
Sorry can't be no more help but a fault that quick of key cycle still makes me believe it's a wiring issue.

TexasBlownV8 said:

+1, that's always an easy way to test for a bad (or wrong) sensor!

Also, which side DID you actually change, passenger or driver side? Bank 2 Sensor 2 is driver side front sensor.

Lastly, it is a heater circuit malfunction, so you could have one of the wires pinched/cut somewhere between the PCM and the O2 sensor. A simple voltage check at the +12 heater pin on the o2 sensor would tell you if voltage is missing or present. (but, since you're only getting 1 code, this is probably not your issue).

Click to expand...

I believe sensor 1 is always the front sensor, sensor 2 is behind the cats.

jhunt47 said:

While a tech for ford for 12 years pcm where always a last result. In my younger years at ford a few pcm got installed that didn't fix the problem. After that I always ohm circuit wires from the pcm to suspect connector ends. Even then once I thought I found a problem would load test that circuit.
Sorry can't be no more help but a fault that quick of key cycle still makes me believe it's a wiring issue.

Click to expand...

Yes, I feel PCM is the last result as well, and I don't really feel that is the source of the problem; I'm just put of ideas and so is everyone I have spoken to, including technicians.
I can't find any obvious wiring problems, and all fuses and relays have tested as good.

slow06stang said:

I had a similar issue using universal o2 sensors. I would ohm check the wires first. Did you check the car side harness to make sure none of the plastic melted and the male side of the o2 sensors are seating properly? Or maybe a wire stretched and broke inside of the sheathing.

Click to expand...

Checked the harness all the way to the PCM; no burns or abrasions anywhere, everything is routed as it should be
The O2 sesnor is getting voltage and is the signal wire is showing that it is switching.
How do you ohm check the harness?

Today we put a pressure gauge on the rail to confirm pressure. It shows 28-29 at idle with vacuum, 39.5 without. Tried a new pressure sensor just to if that could be a source, but there was no change in idle psi or how the car runs or idles. Also the same code p0155 code triggered almost immediately after resetting it with the new pressure regulator.
Free revving it a little, a friend said that there is a slight amount of black smoke from the driver's side pipe (B2).

I may need to take it to a specialist or to Ford. I'm really against a wall here

When you say checked the harness did you check resistance?
If so did you load test the circuit remember even if 1 copper strand is intacked it will still show resistance.

Sorry with a volt meter.

Very good and simple video.

Good video, but that's just for testing the O2 itself. I've used three different O2 sensors (OEM Ford, 2 NTK) all of which threw the same code almost immediately. The problem has to be on the car side

i have confirmed the sensors are getting voltage and the ECU is receiving voltage switching data from it (verified via live data on scanner).
Does anyone know what else I can test today? If nothing else, I'm trying to minimize the diagnosing expense at a shop by handling some stuff beforehand

Maybe someone else can answer this question for some insight. Would a leaking exhaust manifold (gasket or manifold)affect the heater circuit reading? If you have all of the correct voltage and fuel and spark maybe get a cheap ir thermometer and check different parts of the exhaust system on both sides to see if the temps are similar.

slow06stang said:

Maybe someone else can answer this question for some insight. Would a leaking exhaust manifold (gasket or manifold)affect the heater circuit reading? If you have all of the correct voltage and fuel and spark maybe get a cheap ir thermometer and check different parts of the exhaust system on both sides to see if the temps are similar.

Click to expand...

Confirmed no exhaust leaks (stock manifolds and h-pipe)
I have not tested temp via a thermal gun yet, but I'm not sure what that would tell me other than one bank might be leaner than the other.
FWIW, the live data shows the cat temp via the sensors is within spec