P2104 Forced Idle After PCM Swap

[PerfectionMatt]

Folks,

Encountered an issue after swapping the PCM on my 2008 coupe.

Details as follows:

-Swapped in the Mopar PCM 2 years ago. Preloaded by Prefix. I did the throttle relearn using an Autel MD808Pro scanner which I ordered because Prefix recommended it as the only lower end scanner they were aware of that would map the throttles. Worked like a charm, car ran great. My understanding from Prefix was once this was mapped I could swap bank and forth between the Mopar off road unit and the factory PCM with no remapping. My searches of the forums seem to validate this.

-Now I have swapped back in my factory PCM. Started right up, good to go, went to take a drive and no throttle. Steps I have taken:
1. Thinking maybe the throttle relearn in fact has to be redone when swapping back, I used the same scan tool to map the ETC and it seemed to go through the process like the first time just fine. Still no throttle.
2. Tried the above again. No throttle.
3. See that it is showing P2104 forced idle code. Clear code, cycle key to try throttle relearn again- code comes back. Do relearn again anyway- no dice.
4. Disconnect battery for 15 mins- no change.
5. Check bat voltage- seems a little low- figure from all my key on diagnostics. But bat getting old anyway so figured I would swap in a new one. No change.
6. Swap back in the Mopar PCM- fires right up and have throttle.

Other details:
A. I have a battery disconnect on the car- and was interrupted several times while originally swapping the PCM. Now I question did I have the power off when swapping....is PCM damage possible if I did not....I am not 100% sure I did this but now am questioning it.
B. Scanner picks up throttle pedal position, I. E. it sees the input from the pedal 0-100%
C. When doing the throttle relearn attempts I hear the throttle bodies actuating and it seems to be doing exactly what it did when I swapped the Mopar in

Does anyone know what triggers the forced idle code? Anything besides the ETC system? Any other opinions on what I have going on? Should I be able to swap between the mapped PCMs without any scanner work?

Thanks,
Matt

 

[Goggles Pizano]

Check to see if the throttle body blade actually moves when you press the throttle. You are reading the throttle position of the pedal as you indicated but is the blade moving?

 

[99RT10GTS]

Why did you want to put the stock PCM back it? Emissions testing? If you swap back in the Mopar PCM, does it run like it should?

 

[PerfectionMatt]

Goggles,

99% sure the throttle blades are not moving. They do not move ever without the engine running (exception- they do when doing the throttle relearn and I can hear them moving) and with the engine running I cannot see, by myself today. I could get someone to watch later to confirm but I am thinking if I have the forced idle code and no throttle response I am thinking the blades can't be moving. As indicated the scanner recognizes the throttle pedal position input and that seems in order.

99RT10,

Yes, emissions test. When I swap back in the Mopar PCM it runs fine. So I am thinking has to be something with the PCM.

Had another thought- does anyone know if a mileage discrepancy between the BCM and PCM could cause this? The reason I ask is when I swapped in the Mopar originally the mileage was off (Prefix programmed it correctly but I had to move the car unexpectedly so off by about 12 miles??). So in other words when I actually installed the Mopar the mileage did not match the BCM- so I put it in and thought I would try it- and no issues. But perhaps now that I am going back to stock the PCM is not happy for some reason? From research on the forums I do not see anything that would make this a problem running wise. Maybe I will email Prefix and ask their opinion.

 

[GTS Dean]

Your ODO and PCM mileage will vary slightly. They are more of a reality check between the two.

 

[PerfectionMatt]

Thanks all of you for your input.

 

[Dan Cragin]

Send your PCM to Prefix and have them check it. They might need to reload the stock calibration

 

[PerfectionMatt]

Update on this situation:

I contacted Prefix and they suggested that if I did indeed connect the PCM hot I probably lost the throttle calibration. I was able to find a Chrysler dealer that was willing to try doing the relearn and after visiting today it was successful. If it had not worked my next step would have been sending it to Prefix for inspection as Dan suggested.

Thanks for everyone's input.