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Discussion Starter · #1 ·
2001 Silverado 3500, Duramax Diesel LB7, dual fuel tanks. Problem happened suddenly. The fuel gauge always reads empty and low fuel alert is showing. The sending units (dual tanks) are new. Both are AC Delco. When I ohm test the wires from the sending units at the PCM, both correctly read around 40 ohms when empty and around 200 ohms when full. Or, almost full, I only had so much fuel on me. When I use a scan tool to monitor fuel level, it correctly reads the level (%). I siphoned fuel out and the reading went down. I refilled the fuel and level % went up. When I unplug the cluster and reconnect it with the key on, the fuel gauge does an instant sweep to full and sweeps back to empty. So, I think the stepper motor is ok. The needle hadn't been acting up before this problem suddenly happened. Is there something in between the PCM and the cluster that can go wrong?
 

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Solved: There was a fault P1172 fuel pump insufficient flow. This is related to the fuel transfer pump that transfers fuel from the rear tank to the front tank. The ECU turns on the transfer pump when the fuel level of the rear tank is higher than the front tank. If the transfer pump runs for more than 2 minutes and the ECU doesn't see the front tank's level rise and the rear tank's level lower, it trips this fault and disables the fuel gauge, sending it to empty. In my case, the connector for the transfer pump had disconnected.

I thought the fault was related to the CP3 injection pump because I know this CP3 is on its way out. Once I cleared the check engine light, the gauge worked again, for a couple of minutes. Scanned faults and the only fault that came back was P1172. I figured, how does it know about any CP3 issues when the engine wasn't even running? Looked up the code and found out it's related to the fuel TRANSFER pump, not injection pump.
 
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