2008 Chevrolet Impala SS 5.3L - Engine Won't Start with P0641 Code: Sensor Reference Voltage A Circuit Open

A great starting point. Are there any other diagnostic trouble codes (DTCs) present that might indicate a specific sensor failure? Below are relevant details from ALLDATA.com: DTC P0641 – 5V Reference 1 Circuit This circuit supplies 5V to multiple sensors, including: - Manifold Absolute Pressure (MAP) - Fuel Tank Pressure (FTP) Sensor - Accelerator Pedal Position (APP) Sensor 2 - Engine Oil Pressure (EOP) Sensor - A/C Refrigerant Pressure Sensor - Camshaft Position Sensor DTC P0651 – 5V Reference 2 Circuit Provides power to: - APP Sensor 1 - Throttle Position Sensor 1 and 2 - Crankshaft Position Sensor To locate the fault, you can disconnect each component in the affected 5V reference circuit one at a time while monitoring the scan tool. The DTC should change from 'Fault' to 'OK' when the faulty source is removed. If no improvement occurs after disconnecting all components, the issue may lie within the wiring harness.

No additional DTCs were detected. The absence of a P0651 code suggests that the fault is not in the Throttle Position Sensor or Reference-2 circuit. Given this, it's likely one of the sensors on the 5V Reference-1 circuit—such as MAP, oil pressure, or A/C pressure—is at fault.

I'm making progress with this diagnosis. My scan tool captures live data and shows sensor activity, but does not display a clear 5V reference signal to ground that I recognize. Following the AllData wiring diagrams and pinouts, I performed these steps: 1) Disconnected sensors near the TPS and on the same Reference-1 circuit (APP, MAP, Oil Pressure, A/C Pressure), leaving the FTP and Camshaft Position sensors connected due to inaccessibility. 2) Used a digital multimeter (DMM) to measure voltage between +5V and ground at the Accelerator Pedal Position sensor with the key on—read 5 volts. 3) When I reconnected other sensors, the reference voltage dropped to 2.8 volts when the Oil Pressure Sensor was plugged in. 4) After turning off the ignition, I unplugged the oil pressure sensor and reconnected all others. Upon turning the key back on and clearing DTCs, the fuel pump activated and the engine started—though the oil light flashed rapidly and triggered an alarm. I have since replaced the oil pressure sensor and no further issues have occurred. This confirms that grounding a spark tester to an intake manifold bolt (which is electrically connected to the oil pressure sensor harness) caused a short circuit in the 5V reference circuit, effectively blowing the oil pressure sensor. I am relieved that the PCM was not damaged. This case serves as a valuable reminder: always verify grounding points and understand shared electrical circuits before performing diagnostic tests.

I wanted to update this thread—though it's been over a year since the issue occurred. Apologies for the delayed follow-up! Replacing the oil pressure sensor fully resolved the problem, and I have not experienced any recurrence of DTCs or starting issues since. In summary: grounding a spark tester to an intake manifold bolt that is electrically connected to the oil pressure sensor harness caused a short circuit in the 5V reference circuit. The bolt does not screw into the engine block but instead relies on a ground path through the sensor wiring—this led to the failure of the oil pressure sensor. Fortunately, no PCM damage occurred. This experience has become a cautionary tale for DIYers performing spark tests without proper grounding verification. I hope this helps others avoid similar mistakes.