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Old 10-10-2011, 08:46 AM   #21
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DTC P0451

SYSTEM DESCRIPTION
The fuel tank pressure (FTP) sensor measures air pressure or vacuum in the evaporative emission (EVAP) system. The control module supplies a 5-volt reference and a low reference circuit to the FTP sensor. The FTP sensor signal voltage varies, depending on EVAP system pressure or vacuum. The controller uses this FTP signal to determine atmospheric pressure for use in the engine-off small leak test, P0442. Before using this signal as an atmospheric reference, it must first be re-zeroed. If the FTP signal is out of range during the re-zero procedure, this DTC will set.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P0451 Fuel Tank Pressure (FTP) Sensor Performance

CONDITIONS FOR RUNNING THE DTC


DTC P0451 runs only when the engine-off natural vacuum small leak test, DTC P0442, executes.
The number of times this test runs can range from 0-2 per engine-off period. The length of the test can be up to 40 minutes .
CONDITIONS FOR SETTING THE DTC
This DTC will set if the controller is unable to re-zero the FTP sensor voltage within a calibrated range during the engine-off small leak test, P0442.

ACTION TAKEN WHEN THE DTC SETS


The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
CONDITIONS FOR CLEARING THE MIL/DTC


The control module turns OFF the 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 DTC with a scan tool.
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, this code can set. When pressure is applied to the system and released, a properly operating system will return to the atmospheric pressure rapidly. By using a scan tool and the J 41413-200 Evaporative Emission System Tester (EEST), pressure can be applied to the system, then released, while monitoring the FTP sensor parameter to see that pressure can be released within 30 seconds .
An FTP sensor that is skewed or does not have a linear transition from low to high may cause this code to set. Scan tool output controls, snapshot, and plot functions can help detect erratic sensor response. To test the sensor signal under vacuum conditions, use the Quick Snapshot and the Purge/Seal functions to capture data while commanding purge to 20 percent , then plot the data to look for erratic sensor operation. A similar test can be done for the pressure side of the sensor operation by applying pressure with the J 41413-200 while taking a snapshot.
A full fuel tank may cause misdiagnosis.
When using the electronic emission system tester (EEST) to apply pressure, you can regulate the amount of pressure by activating the remote switch ON and OFF while observing pressure in the EVAP system using a scan tool.

sounds like it could be your sensor, canister, or clog in the lines for the canister.
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Old 10-10-2011, 12:56 PM   #22
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This sounds like it is going to be a fishing expadtion. There is no way to tell if those things are bad besides just replacing them like I did the Evap Valve...

By using a scan tool and the J 41413-200 Evaporative Emission System Tester (EEST), pressure can be applied to the system, then released, while monitoring the FTP sensor parameter to see that pressure can be released within 30 seconds .


I sure don't have those tools... Looks like I will have to take it into the shop.
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Old 12-01-2012, 01:08 PM   #23
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Jamala00 did you ever get this figured out?
I have the same thing with my 07 Tahoe. Just replaced the evap and about to replace the gas cap. The only thing I have different is that I can not put gas in it. It'* like it vapor locks and shuts the pump off and then spits fuel back out the filler neck. The filler neck, vent hose, and everything else has been checked for a blockage. Any ideas?
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