HELP! Troubleshooting guru needed: P0155 will not go away

UPDATE:

Low, weak idle problem solved, and most of the weak running situation cleared up

I started questioning whether I had reset the ECU properly, so I went through the steps very by the letter
1. Reflash the tune to stock
2. Disconnect negative terminal of battery for at least 30 minutes (I suggest 1 Hour)
3. Here's the critical one I messed up; crank the car and let it idle until it reaches full operating temperature, and then continue to let it idle for another 3 minutes. This means you cannot touch the throttle at all, and must wait for the cooling fans to kick on. My car has a heavily upgraded cooling system, so in 72 degree Los Angeles weather with the hood down and zero frontal airflow, it took about 27 minutes of idling for the fan to kick on. At this point, most instructions say give it at least another minute of idle time, I suggest three minutes.
4. Turn on the A/C, allow to idle for at least another minute. Again, you cannot touch the throttle, and I gave it three minutes of idling. Idle should noticeably normalize in this time period.
5. Turn A/C off, allow to idle another minute, then take the car for a drive in varying conditions.

This resulted in a better crank than I've had in a long time, and a normal, strong idle with and without A/C. After the ECU learned a bit, the engine also ran much stronger.


The bad news; p0155 still came back. Last resort will be a new Motorcraft DY1039 O2 sensor before taking it to a shop.

Still love to hear your ideas as I will have the weekend to troubleshoot again

Horsepower67 said:

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

Click to expand...

Validation of car chassis wiring directly related to the O2 2-1 Heater circuit. You may wish to revisit the wiring based on the P0155 code coming on immediately at startup as another has stated. Here is a method that should help.


First is to check that 12vdc is getting to the O2 sensor from the PCM relay power bus via F47.

1.) Disconnect O2 bank 2-1 sensor from C172 connector on wiring harness.
2.) set up meter to read volts DC.
3.) Connect black lead to good chassis ground.
4.) Turn ignition switch to KEY ON - ENGINE OFF position.
5.) Verify 12vdc at C172-2 red/yellow stripe wire. BE VERY CAREUL NOT
TO TOUCH RED LEAD PROBE TO ANY GROUND OR OTHER PIN IN THE CONNECTOR DURING PROCESS OR F47 MAY
BLOW.

a.)If 12vdc present, move to next series of checks.
b.) If no 12vdc present locate open wire in harness
between Splice S101 and Connector pin C172-2 and repair Put car
back in order and start to see if P0155 goes away.

This verifies the 12vdc source and wire feeding the OS sensor heater.


DO NOT ATTEMPT THE FOLLOWING WITH THE BATTERY CONNECTED!!! DISCONNECT!!! BEFORE BEGINNING!!!!

Shut ignition key off and remove keys. Disconnect Battery power and isolate.

Disconnect PCM connector C175B at the PCM.

Verify/Disconnect C172 from O2 sensor Bank2 -1.

The following checks look for open wire AND short to chassis ground wires.

SHORT TO CHASSIS GROUND WIRE CHECK

1.) set voltmeter to read resistance.
2.) connect black lead to known good chassis ground.
3.) place red lead on C172-1 (yellow/light blue stripe).
a.) meter should read OL or very high resistance. IF YES GO TO 4.
b.) IF NO LOCATE SHORT TO CHASSIS GROUND ON WIRE AND
REPAIR. PUT CAR BACK IN ORDER AND SEE IF P0155 goes away.

OPEN (BROKEN) WIRE CHECK

4.) While under car, take an alligator clip and connect to C172-1 (yellow/light blue stripe) and the other end of the alligator clip to solid chassis ground.
5.) AT PCM in engine bay place black lead to good chassis ground. Place red lead to C175B -70 (Yellow/Lightblue stripe). Observe a resistance reading of less than 2 ohms.
a.) If less than 2 ohms seen, wire is good.
b.) If resistance reads OL or high resistance, locate broken wire
between C175B-70 and C172-1 and repair. Put car back in order and
start to see if P0155 code stays clear.

These checks verify an open wire to ground and short to ground.

There is really no reason to discuss F47 being blown. It is likely not. Other codes would also pop up as well, but you can for redundancy sake.

Hope this helps you out.

As SPOCK said in Star Trek, "once you eliminate the possible only the impossible remains." So based on that the only other thing I would think of but is likely not related to this code would be a clogged cat on the driver's side. Your foot would get extremely warmer than normal very quickly in like 10-15 minutes and you would feel a distinct difference in exhaust flow between the right and left tail pipes in spite of the H-pipe. Also the drivers side Cat will be very dissimilar in color to the passenger side if only the one is blocked. Usually happens from heat over time from exhaust.
Good luck

Fantastic post, thank you for taking the time to write this up so clearly.

I've already done the first test and confirmed 12V in the harness, so I guess I'll be trying the second this weekend. Hopefully, it yields something- and doesn't reset the PCM again

boomer9999 said:

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

Click to expand...

That is correct. However, Bank 1 is Passenger Side and bank 2 is Drivers Side. That gets mistaken a lot.

Horsepower67 said:

Fantastic post, thank you for taking the time to write this up so clearly.

I've already done the first test and confirmed 12V in the harness, so I guess I'll be trying the second this weekend. Hopefully, it yields something- and doesn't reset the PCM again

Click to expand...

No problem. Tried to keep it straight forward. Hard to know what level of understanding people have with voltmeters and electrical troubleshooting. Once you have verified your wiring, then we can look at bigger fish. No reason to point a finger at a high ticket item until other things are eliminated.

Had a problem like this when I replaced my auto trans. Had to take exhaust pipes off to install Trans and when putting pipes back on unknowingly dinged the back of a oxygen sensor where the wire goes into it. The metal at this point was bent over into the wire shorting it. Car ran like crap. Had a hard time to find the problem. Once found (luckily) I bent the metal back off the wire, used some marine gunk to seal up the slit. That was three years ago and the car has run great since then.

Hi everyone,

Reviving this thread with an update for everyone. The car was fixed in Dec 2016, and the problem ended up being exactly what I feared from the beginning; fried PCM.

I ordered a reman one from Flagship One since they come loaded with the stock tune and have a lifetime warranty. $569 vs. $1,500 new through Ford, and a much safer bet than a junkyard unit. Definitely recommend them if you are ever in need; they were very helpful and polite and the PCM shipped quickly.

Unfortunately, the new PCM also requires a trip to Ford (or a shop with the Ford specific software) to have the anti-theft and keys programmed to accept the PCM, which will run another $350 or so.

Long story short, car runs great now with no lights or issues.

^^^ Sorry to hear. The local speedshop I deal with has full capability to fully reflash replacement PCM's (PATS KEYS AND ALL) and has done so for an acquaintance of mine who had his crap out recently. You may wish to look into that before just rolling over to Ford Service for that kind of cash.