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Got an 07 Impala with the 3.9 that's got me scratching my head.
It has an intermittent PO641, with reduced engine power, service TCS, etc. messages. I have not been able to reproduce the condition reliably.
I have checked over the harness very closely in the areas that are outlined in the TSBs, and also pulled the power steering pump to check the CPS plug.
The few times I have been able to replicate the condition, it is only with the engine running. KOEO 5V Rev bus 1 voltage is perfect, if you start the engine it shoots up to 7-8 volts, shut the engine off and it goes back to 5.
Since it only acts up with the engine running, I cannot simply turn the key on and start unplugging components to isolate the fault.
At this point I'm thinking bad PCM, but I do not know of a way to confirm it.
I’d check powers and grounds at pcm make sure one isn’t out of spec. Here’s a idea but could you cut the 5 volt ref 1 wire at the pcm then the other side splice into 5 volt ref 2 then see if issue still is present.
Unfortunately that 5V ref bus 1 consists of 7 individual wires that are tied together within the PCM, would be a lot of work and damage to the harness to splice them onto bus 2.
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