Code P0651 is but how can I find out what is wrong?

DTC P0651

Circuit Description
The powertrain control module (PCM) provides 5 volts to the following sensors:
The air conditioning (A/C) pressure sensor
The fuel tank pressure (FTP) sensor, if equipped
These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects that the voltage is out of tolerance, DTC P0651 sets.

Conditions for Running the DTC
The engine is running.

Conditions for Setting the DTC
The PCM detects a voltage out of tolerance condition on the 5-volt reference circuit.
The above condition is present for longer than 2 seconds.

Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.

DTC P0651 Step Action
System Check 1 -- Engine Did you perform the Diagnostic System Check-Engine Controls?
Go to Step 2

1.Observe the Freeze Frame/Failure Records for this DTC. 2.Turn OFF the ignition for 30 seconds. 3.Start the engine. 4.Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Does the DTC fail this ignition cycle?
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Go to Intermittent
Conditions
3 Is this vehicle equipped with a fuel tank pressure sensor? - Go to Step 4 Go to Step 5 1.Turn OFF the ignition. 2.Disconnect the air conditioning (A/C) pressure sensor. 3.Turn ON the ignition, with the engine OFF. 4.Measure the voltage from the 5-volt reference circuit of the A/C pressure sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems. 4 4.8-5.2 V
Is the voltage within the specified range? 1.Turn OFF the ignition. 2.Disconnect the air conditioning (A/C) pressure sensor. 3.Turn ON the ignition, with the engine OFF. 4.Measure the voltage from the 5-volt reference circuit of the A/C pressure sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.
Go to Step 3
Go to Step 6 Go to Step 7
5
4.8-5.2 V

6
Is the voltage within the specified range? Go to Step 15 Go to Step 8 1.Connect the A/C pressure sensor. 2.Disconnect the fuel tank pressure (FTP) sensor. 3.Measure the voltage from the 5-volt reference circuit of the FTP sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems. Go to Intermittent 4.8-5.2 V
Is the voltage within the specified range?
Conditions Go to Step 15
7 Is the voltage measured in the previous step more than the specified value? 5.2 V Go to Step 11 Go to Step 9 8 Is the voltage measured in the previous step more than the specified value? 5.2 V Go to Step 12 Go to Step 10 Monitor the DMM while disconnecting the FTP sensor. Does the voltage return to within the specified range when the FTP is disconnected? Go to Step 14 Go to Step 10 1.Turn OFF the ignition. 2.Disconnect the powertrain control module (PCM). 3.Test the 5-volt reference circuit for a short to ground or any sensor low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. 9 10 -4.8-5.2 V
Did you find and correct the condition? Monitor the DMM while disconnecting the FTP sensor. Does the voltage return to within the specified range when the FTP sensor is disconnected? Test all 5-volt reference circuits for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? Test the FTP sensor signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?
Go to Step 17 Go to Step 16
11
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Go to Step 13 Go to Step 12
12
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Go to Step 17 Go to Step 16
13
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Go to Step 17 Go to Step 16
14
Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement
(1500 Series) or Fuel Tank Pressure

15
16
17
Did you complete the replacement? Go to Step 17 -Replace the A/C pressure sensor. Refer to Air Conditioning (A/C) Refrigerant -Pressure Sensor Replacement in Heating, Ventilation, and Air Conditioning. Did you complete the replacement? Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement . Did you complete the replacement? 1.Clear the DTCs with a scan tool. 2.Turn OFF the ignition for 30 seconds. 3.Start the engine. 4.Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. --Go to Step 17 -Go to Step 17 -
18
Did the DTC fail this ignition? Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed?
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Go to Step 2 Go to Step 18 Go to Diagnostic Trouble Code (DTC) List System OK