Chevy colorado getting a PO651 code, "sensor reference circuit b open/short" we have tried replacing the bat. and checking sensor functionality. engin is very clean so no rusted or broken wires. truck wont start. will turn over but not start.

Hi there:

DTC P0641 - 5-Volt Reference 1 Circuit

DTC P0651 - 5-Volt Reference 2 Circuit

Diagnostic Fault Information
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit/System Description
The engine control module (ECM) has 2 internal 5-volt reference busses called 5-volt reference 1 and 5-volt reference 2. Each reference bus provides 5-volt reference circuits for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to that reference bus. The ECM monitors the voltage on the 5-volt reference busses.

The 5-volt reference 1 bus provides 5 volts to the following sensors:
* The manifold absolute pressure (MAP) sensor
* The fuel tank pressure (FTP) sensor
* The air conditioning (A/C) refrigerant pressure sensor
* The accelerator pedal position (APP) sensor 2
* The intake camshaft position (CMP) sensor

The 5-volt reference 2 bus provides 5 volts to the following sensors:
* The APP sensor 1
* The throttle position (TP) sensor 1 and 2
* The crankshaft position (CKP) sensor

Conditions for Running the DTC
* DTCs P0601, P0602, P0603, P0604, P0605, P0606, P0607, and P2610 are not set.
* The ignition is in Unlock, Accessory, Run, or Crank.
* The ignition voltage is more than 5.23 volts.
* DTCs P0641 and P0651 run continuously when the above conditions are met.

Conditions for Setting the DTC
The ECM detects a voltage out of tolerance condition on the 5-volt reference 1 or 2 bus for more than 0.5 second.

Action Taken When the DTC Sets
DTCs P0641 and P0651 are Type A DTCs.

Conditions for Clearing the MIL/DTC
DTCs P0641 and P0651 are Type A DTCs.

Diagnostic Aids
It may be possible to locate the fault by disconnecting one component at a time from the 5-volt reference circuit while viewing the 5-Volt Reference circuit parameter on the scan tool. The scan tool parameter would change from Fault to OK when the source of the fault is disconnected.

If all 5-volt reference components have been disconnected and a Fault is still indicated, the fault may exist in the wiring harness.

Circuit/System Verification
With the ignition ON and the engine OFF, observe the 5-volt Reference 1 and 2 parameters. The scan tool should display 4.3-4.7 volts for a normally operating system.

Circuit/System Testing
Important: Additional DTCs will set when disconnecting the components.

1. Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC.

Important: A short to voltage on the signal circuit of certain components may cause this DTC to set.

2. Ignition ON, measure for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.

* If the voltage is less than the specified value, test for a short to ground on all the 5-volt reference circuits. If all circuits test normal, replace the control module.
* If the voltage is greater than the specified value, test for a short to voltage on all the 5-volt reference circuits, or the signal circuit, for each component associated with the appropriate 5-volt reference circuit. If all circuits test normal, replace the control module.

3. Connect each component associated with the 5-volt reference circuit, one at a time, while monitoring the voltage. The voltage should not change more than 1 volt.

* If the voltage display changes more than the specified value when a component is connected, replace the component.


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