Diagnostic Aids

A restriction in the EVAP canister or vent lines could prevent fuel vapor pressure from bleeding off fast enough. If the vent system cannot bleed off pressure fast enough, the re-zero procedure may not complete successfully, which could cause this code to set.
By using the purge solenoid command on a scan tool, vacuum can be applied to the system instead of pressure. With the EVAP canister vent solenoid valve open and the EVAP canister purge solenoid valve commanded to 100 percent, the vacuum should not increase to more than 6 inches H2O.
An EVAP canister filter that is restricted can cause this DTC to set. Refer to Evaporative Emission (EVAP) Canister Filter Replacement .
For intermittent conditions, refer to Intermittent Conditions .
Ensure that the reference port on the FTP sensor is unobstructed.

DTC P0451

Step	Action	Values	Yes	No
Schematic Reference: Evaporative Emissions (EVAP) Hose Routing Diagram and Engine Controls Schematics
1	Did you perform the Diagnostic System Check - Vehicle?	-	Go to Step 2	Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2	Inspect the Evaporative Emission (EVAP) System for the following conditions: A damaged EVAP canister vent solenoid valve-Refer to Evaporative Emission (EVAP) Canister Vent Solenoid Valve Replacement . A pinched EVAP vent hose A damaged EVAP canister-Refer to Evaporative Emission (EVAP) Canister Replacement . Did you find and correct the condition?	-	Go to Step 12	Go to Step 3
3	Turn OFF the ignition. Disconnect the purge line from the EVAP canister purge solenoid valve. Refer to Evaporative Emission (EVAP) Canister Purge Solenoid Valve Replacement . Turn ON the ignition, with the engine OFF. Is the fuel tank pressure sensor parameter within the specified range?	-1 to +1 in H2O	Go to Step 4	Go to Step 8
4	IMPORTANT: DO NOT exceed the specified value in this step. Exceeding the specified value may produce incorrect test results. Refer to the J 41413-200 Evaporative Emissions System Tester (EEST) operation manual for detailed instructions. See Special Tools . Turn OFF the ignition. Connect the EVAP purge pipe. Install the J 41415-40 Fuel Tank Cap Adapter or GE-41415-50 Fuel Tank Cap Adapter to the fuel fill pipe. Use the J 41413-200 to pressurize the EVAP system with NITROGEN to the first specified value. See Special Tools . Command the EVAP canister vent solenoid valve open. Does the Fuel Tank Pressure Sensor parameter return to the second specified value?	5 in H2O -1 in H2O to +1 in H2O	Go to Step 5	Go to Step 7
5	Start the engine with the J 41413-200 still connected to the fuel filler neck. See Special Tools . Allow the engine to idle. Use the PURGE/SEAL function to seal the system, with a scan tool. Command the EVAP canister purge solenoid valve to 30 percent. Observe the VACUUM/PRESSURE gage on the J 41413-200 and the FTP parameter on the scan tool. See Special Tools . Allow the vacuum to increase on the gage of the J 41413-200 , until it reaches approximately 16 inches H2O or until the vacuum reached the abort limit on a scan tool. See Special Tools . Use the PURGE/SEAL function to seal the system, with a scan tool. Was the difference between the FTP parameter on a scan tool and the VACUUM/PRESSURE gage on the J 41413-200 less than the specified value?. See Special Tools .	1 in H2O	Go to Step 6	Go to Step 8
6	Did the FTP parameter on a scan tool display more than the specified value?	3.2 V	Go to Diagnostic Aids	Go to Step 10
7	Inspect the EVAP vent system for a restriction. Did you find and correct the condition?	-	Go to Step 13	Go to Diagnostic Aids
8	Test for an intermittent and for a poor connection at the fuel tank pressure (FTP) sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 12	Go to Step 9
9	Test the low reference circuit of the FTP sensor for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 12	Go to Step 10
10	Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement?	-	Go to Step 12	-
11	Replace the EVAP canister vent solenoid valve. Refer to Evaporative Emission (EVAP) Canister Vent Solenoid Valve Replacement . Did you complete the replacement?	-	Go to Step 12	-
12	Turn OFF the ignition. Disconnect the purge line from the EVAP canister vent solenoid valve. Turn ON the ignition, with the engine OFF. Is the fuel tank pressure sensor parameter within the specified range?	-1 to +1 in H2O	Go to Step 13	Go to Step 2
13	IMPORTANT: DO NOT exceed the specified value in this step. Exceeding the specified value may produce incorrect test results. Turn OFF the ignition. Reconnect all disconnected components. Connect the J 41413-200 to the fuel fill pipe. See Special Tools . Turn ON the ignition, with the engine OFF. Command the EVAP canister vent solenoid valve closed with a scan tool. Turn the nitrogen/smoke valve on the J 41413-200 control panel to NITROGEN. See Special Tools . Use the remote switch to pressurize the EVAP system to the first specified value. Observe the fuel tank pressure sensor in H2O with a scan tool. Command the EVAP canister vent solenoid valve open with a scan tool. Is the fuel tank pressure sensor parameter less than the second specified value?	5 in H2O 1 in H2O	Go to Step 14	Go to Step 2
14	Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed?	-	Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information	System OK