P0641 Limp mode sucks....

[Bildo]

So I got a P0641 code on my car, I suspected last year that I may have a flakey ECM due to a ground issue I had that was causing intermittent issues (which may have damaged the ECM). I was getting on it a bit hard and threw a code putting it into limp mode. I have unplugged all these sensors one at a time and still have the code. Can't visibly see any shorts or bare wires anywhere. Key on engine off in VCM scanner I am seeing my map and oil pressure reading maxed out high. Purchased a used ECM out of salvage car of the same year and will be swapping it out today to see if the Key on engine off reading on the oil pressure sensor will read zero. Other than that I can't think of anything else to try. The pic here shows the codes I am getting, disregard the fuel codes as those are due to my aftermarket fuel system.

[Bildo]

So I swapped the PCM out no change, unplugged the three sensors throwing codes and measured the hot pin on all of them they all read battery voltage 12+ volts. Kinda making me think there is a short somehwere on the harness.

[G8One2]

Circuit high faults means the circuit is open, not shorted.

[Bildo]

Wondering why I am getting 12V on what is supposed to be a 5V circuit..... I will pull what I can apart on the harness this weekend and see if I can find anything obvious. If now a new harness is less than 250 I will just have to re-pin the flex fuel sensor.

[Bildo]

Made spaghetti out of my harness this weekend to look for any damaged wires, couldn't find a single one. Noticed that the I get 12V constant on the 5V Reference 1 circuit anytime the battery is plugged in. If I disconnect the PCM, the voltage goes away when either X1 or X2 is unplugged. Still at a loss what the heck is happening. Will put the car on jack stands tonight and start looking at the sensors that I can only access from under the car (VSS, Cam position, ect)

[Jayjellyfish212]

Hey on mine I had a very similar issue. Drove me nuts chasing the issue. I had code PO651 sensor reference voltage B. I couldn’t even start the car at all. Kept telling me to push clutch in to start but wasn’t recognized when I was pushing it in. Power reduced mode also . I popped started it and that was no good since the car wasn’t recognizing my clutch being pushed in it would stall. I ended up towing car home and going bonkers for days. After replacing the clutch position sensor turns out that wasn’t the problem (was a PITA to do), I ended up finding my issue was the actual plug connector to the throttle body. Have no clue how this crapped the bed but it did. I replaced it and everything was back to normal.

I found this connector to be the issue because I tried to use my HPT software to do the clutch relearn programming and it wouldn’t even recognize anything. So I and a friend figured there’s a short or a broken wire somewhere. My friend stayed in the car with my laptop and HPT on and I started just wiggling wire harnesses and then he yelled while I was in the engine bay hold up something changed. So I slightly wiggled the wire harness I last touched when he yelled and it was the throttle body connector. My entire car is fully custom wired so I knew what wires were in there and where they went. Upon opening up the harness to look the wires over I found no issues with any of the wires themselves.

The reason I’m putting this out there is we learned that the sensors on these cars are banked together and not individually wired. So that is why I was getting the clutch issue because it’s all banked together each sensor.

Check the throttle body connector real good and then all other connectors real good since you found no broken or messed up wires. Check and clean any grounds to. I’ve seen those get so corroded causing tons of issues. I’m betting you have a issue in one of the connectors that’s just not making good connection like mine was.

[Bildo]

Originally Posted by Jayjellyfish212

Hey on mine I had a very similar issue. Drove me nuts chasing the issue. I had code PO651 sensor reference voltage B. I couldn’t even start the car at all. Kept telling me to push clutch in to start but wasn’t recognized when I was pushing it in. Power reduced mode also . I popped started it and that was no good since the car wasn’t recognizing my clutch being pushed in it would stall. I ended up towing car home and going bonkers for days. After replacing the clutch position sensor turns out that wasn’t the problem (was a PITA to do), I ended up finding my issue was the actual plug connector to the throttle body. Have no clue how this crapped the bed but it did. I replaced it and everything was back to normal.

I found this connector to be the issue because I tried to use my HPT software to do the clutch relearn programming and it wouldn’t even recognize anything. So I and a friend figured there’s a short or a broken wire somewhere. My friend stayed in the car with my laptop and HPT on and I started just wiggling wire harnesses and then he yelled while I was in the engine bay hold up something changed. So I slightly wiggled the wire harness I last touched when he yelled and it was the throttle body connector. My entire car is fully custom wired so I knew what wires were in there and where they went. Upon opening up the harness to look the wires over I found no issues with any of the wires themselves.

The reason I’m putting this out there is we learned that the sensors on these cars are banked together and not individually wired. So that is why I was getting the clutch issue because it’s all banked together each sensor.

Check the throttle body connector real good and then all other connectors real good since you found no broken or messed up wires. Check and clean any grounds to. I’ve seen those get so corroded causing tons of issues. I’m betting you have a issue in one of the connectors that’s just not making good connection like mine was.

Yeah I actually extended my harness in two places one of which was the MAP sensor since it is now in the back of my Holley Sniper Intake and the other was the Throttle body because that was really tight and I did those both very nicely tinning the wires and soldering with multiple layers of heat shrink. I said screw it today and ordered a new harness.

[Jayjellyfish212]

Originally Posted by Bildo

Yeah I actually extended my harness in two places one of which was the MAP sensor since it is now in the back of my Holley Sniper Intake and the other was the Throttle body because that was really tight and I did those both very nicely tinning the wires and soldering with multiple layers of heat shrink. I said screw it today and ordered a new harness.

Hahaha... nice! Yeah you hardly see any of my wires much in the engine bay too. Did you find the harness connector ok? I found an OEM Delphi for mine on EBay for 12 bucks and it was exactly the same wire colors to.

Did you see if that connector is the issue on yours? If you have HPTUNERS on your laptop you could do what we did on mine... have someone in the car with the laptop and key on then wiggle the connector and see if anything jumps around on the perimeters on the laptop. Hopefully this is your issue. If not check all the connectors real good..

[Bildo]

Update, ended up being the circuit for the fuel tank pressure sensor, since I have a squash dual pump system installed, I was able to just snip and cap the wire for that circuit. Pin 13 on the X102 connector. No new harness needed, put it all back together and shut off the codes for the fuel system in HP Tuners. Runs great now no MIL, and no random 12V on the 5v reference circuit.

[Chevy67deuce]

Originally Posted by Bildo

Update, ended up being the circuit for the fuel tank pressure sensor, since I have a squash dual pump system installed, I was able to just snip and cap the wire for that circuit. Pin 13 on the X102 connector. No new harness needed, put it all back together and shut off the codes for the fuel system in HP Tuners. Runs great now no MIL, and no random 12V on the 5v reference circuit.

I have same trouble code and reduced power. I have a backhalfed 2012. Aftermarket fuel cell. What exactly did you turn off to get ride of the p0641 ? What did you turn off in hp tuners?