OBD-II trouble code
P0451: Evaporative Emission System Pressure Sensor/Switch Range/Performance
The fuel tank pressure signal is present but the module does not trust it. On the 2007 Saturn Relay chart, P0451 sets when the sensor cannot be re-zeroed during the engine-off small-leak test.
Quick facts
- System
- Powertrain
- Category
- Emissions / EVAP
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0451 mean?
P0451 is Evaporative Emission System Pressure Sensor/Switch Range/Performance. On the 2007 Saturn Relay chart, and in the 2008 Pontiac Torrent procedure that matches it, this code runs only while the engine-off natural-vacuum small-leak test, P0442, is executing. That test can run from 0 to 2 times per engine-off period, for up to 10 minutes. The code sets if the controller cannot re-zero the fuel tank pressure sensor voltage within a calibrated range during that test.
The same charts say to repair P0452, P0453, or P0641 first if any of those is stored. P0452 and P0453 are the out-of-range electrical codes, under 0.1 volt and over 4.9 volts. A performance code is not diagnosed on top of a circuit code the chart has already named.
The diagnostic aid on those charts is a restriction, not a drifting-sensor percentage. A restriction in the canister or the vent lines can keep vapor pressure from bleeding off fast enough for the re-zero to finish. With the vent solenoid open and the purge solenoid commanded to 100 percent, vacuum should not increase to more than 9 inches of water. The reference port on the sensor has to be clear. The verification is: vent commanded closed, purge at 30 percent, and the sensor voltage scaling smoothly as vacuum rises to the abort limit.
The further test on those charts uses the J 41413-200 evaporative tester at the filler neck, purge commanded to 30 percent, vacuum taken to about 16 inches of water or to the abort limit. If the scan-tool pressure and the tester gage differ by more than 1 inch of water, the chart replaces the sensor. It also replaces the sensor if the displayed parameter is under 3.2 volts during that test. Those figures belong to this procedure.
A loose cap is not the set condition written for P0451. The engine-off test is. Fuel control does not use this sensor to set injector pulse width.
Common causes
- Re-zero failure during the engine-off P0442 test, which is the set condition on the 2007 Relay chart
- P0452, P0453, or P0641 stored with it, which that chart repairs first
- Canister or vent restriction that keeps vacuum from staying at or below 9 inches of water with the vent open and purge at 100 percent
- Blocked reference port on the fuel tank pressure sensor
- Sensor whose reading differs from the J 41413-200 gage by more than 1 inch of water, or whose parameter is under 3.2 volts, on that chart's test
- High resistance in the low reference, which the Relay fault table also associates with P0451
Symptoms
- Check engine light with P0451
- EVAP monitor not completing
- A fuel tank pressure trace that does not scale smoothly when purge is commanded to 30 percent with the vent closed
- No change in fuel control from this sensor
Diagnostic steps
- 1.If P0452, P0453, or P0641 is stored, repair that code first. The 2007 Relay chart says so before the performance test.
- 2.Confirm this vehicle uses the engine-off re-zero test. On the Relay and Torrent charts, P0451 runs only then, up to twice an engine-off period, for up to 10 minutes.
- 3.Command the vent closed and purge to 30 percent. The voltage should scale smoothly as vacuum rises toward the abort limit.
- 4.Check the vent path the aid describes. With the vent open and purge at 100 percent, vacuum should not rise above 9 inches of water. Inspect the reference port.
- 5.If the J 41413-200 is the tool the procedure calls for, compare it with the scan-tool parameter out to about 16 inches of water or the abort limit. More than 1 inch of water of disagreement, or a parameter under 3.2 volts, is the chart's reason to replace the sensor.
- 6.Do not replace the sensor because the cap was loose. That is not the set condition on this chart. Do not treat an at-rest reading as a failure unless the chart's rest value, 1.5 volts cap-off on these two vehicles, has actually been missed. A tank already at atmosphere will not move when the cap is loosened.
- 7.Keep ignition sources away when the cap or the tester is on the filler neck. Turn a warm cap off slowly.
Repair cost
$100 – $600
The low end of the printed bracket is a vent line or a reference port cleared, which the Relay diagnostic aid checks before the sensor. The high end is the fuel tank pressure sensor after the J 41413-200 comparison fails, including the labor to reach it. Read the bracket as a planning span, not a quoted price.
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Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.