2014 GMC 5V Reference 1 fault (p0641)

We have this truck in the shop and I am unsure of what i am seeing.  The 5v ref out of the PCM is a saw tooth or data line.  I am trying to isolate a short or a failed PCM.  My first question is what and why am i seeing a saw or data or not a constant 5v on this?  Is it a function of the protection device in the PCM?  If I watch the screen, I can get a solid 5v for a few seconds and then its back.

Hey, that's a cool waveform! My first thought is that the 5v ref circuit is touching something that gets pulled to ground , like an 02 heater circuit.
I've seen a few full size GM SUVs where the 02 harness and parts of the engine harness touch the front drive shaft and cause 5v ref circuit issues. Other areas known to run through are around back of the driver's side cylinder head and valve cover area.

On these tough issues I us AI to get me started. Here is what chat gtp said
*P0641 = “5V Reference 1 Circuit Open” — very common on GM vehicles when there's a short to ground or sensor fault on the shared 5V reference bus.


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? Your Observation: Sawtooth or Data-Like 5V Line

What you're seeing — a fluctuating or “sawtooth” waveform on what should be a flat 5V — is not normal. The 5V ref circuit should be a clean, constant DC voltage.

If it drops out periodically or looks like digital chatter, that typically means:

? 1. The PCM is trying to maintain 5V but a component is intermittently pulling it down.

Short to ground (intermittent or resistive)

Sensor internal failure (e.g. MAP, A/C pressure, TPS, crank sensor)

Wire chafing — especially near exhaust, brackets, or engine harness near the intake



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⚡️ Why a Sawtooth / Digital Look?

Yes — you're right to ask if this is part of a protection circuit — many PCMs use a current-limited driver for the 5V line. When a fault occurs:

The PCM may pulse or cycle the 5V rail internally (as a form of current-limiting or retry logic).

This creates what looks like choppy voltage, like it’s trying to “poke” 5V out and backing off.

If you disconnect the sensors and still see it, it may be internal to the PCM.

But if disconnecting one specific sensor stabilizes it, that sensor (or its wiring) is likely the short.



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? Diagnosis Strategy

Start by identifying everything on 5V Reference 1 in that truck. On a 2014 GM truck, 5V Ref 1 usually feeds:

MAP sensor

A/C pressure sensor

Fuel tank pressure sensor (EVAP)

APP sensor (some models)

EGR or intake cam solenoid (if equipped)


? Step-by-step:

1. Unplug all 5V sensors (key on, engine off).


2. Check for stable 5V at the PCM terminal or a good known sensor plug.

If it goes stable, plug sensors back in one at a time until it drops again.



3. If still unstable with all unplugged, test 5V at the PCM pin itself.

If it's bad at the pin → likely a failed PCM.

If it’s clean at the PCM but not at the harness → wiring short in the loom.





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? Tools That Help

Scope: Your sawtooth pattern confirms instability — good catch.

Amp clamp on 5V line can help find current surges when the line drops.

T-pin + voltmeter at PCM to verify if signal is clean internally.



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? Tech Note

GM PCMs do NOT intentionally put digital comms on 5V ref lines — so any data-like chatter is a side effect of the protection trying to maintain voltage under fault. It's not CAN, UART, or LIN, and should never carry modulated signals.


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? Summary

A clean 5V reference should be solid, not sawtooth.

The sawtooth indicates either a PCM internal current limit cycling, or a shorted load pulling it down repeatedly.

Unplug each 5V sensor on that circuit until the line stabilizes.

If none make it go clean, check the PCM pin directly — likely PCM fault if bad there.


Let me know which sensors are on that 5V circuit if you want help narrowing the scope faster.