Error Code P0451 is defined as Evaporative Emission Control System Pressure Sensor Range/Performance. This error code indicates a change of value in the pressure that is outside the specifications set by the manufacturer, during the operation of the vehicle or EVAP monitor test.

This code is a generic trouble code, which means it applies to all vehicles equipped with OBD-II made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another. This code usually appears in Audi, Chevrolet, Dodge, Ford, Ram, GMC, Toyota, and VW.

Definition

The EVAP system uses a venter reservoir called ‘canister’, which job is to store excessive fuel vapors until the engine is being run under the right conditions to burn them properly. This reservoir keeps the fuel vapors from escaping to the atmosphere.

The EVAP system is a complex system of rubber hoses and metal tubes that stretches from the fuel tank, up to the engine compartment. Some of its components include the vent control valve/solenoid, the purge valve/solenoid, the EVAP pressure sensor, and the charcoal canister.

When the fuel is stored, a fuel vapor pressure is created will act as a propellant, causing the vapor to go through the network of rubber hoses and metal tubes, and will reach the canister. These fuel vapors from the tank storage are then discharged through the pressure relief valve (located at the top on top of the fuel tank), which is then forced through a complex web of rubber lines and metal connectors to the charcoal canister, where they will accumulate. The canister contains a charcoal element that absorbs fuel vapors, storing them for release at the right time.

Basically, the job of the purge control valve/solenoid is to regulate the intake vacuum to the EVAP canister and draw fuel vapor to the engine when the condition is ideal for the fuel to be burned, rather than releasing them to the atmosphere. The PCM (powertrain control module, also known as ECM or engine control module) control the purge control valve/solenoid, which is the heart of the EVAP system.

If the PCM detects that the EVAP and/or fuel tank pressure is operating outside the specified range over a set period of time, and under certain circumstances, the Error Code P0451 is set and activate the Check Engine light.

Other related EVAP emission trouble codes include P0450, P0452, P0453, P0454, P0455, P0456, P0457, P0458, and P0459.

Common Symptoms

As with most EVAP emission trouble codes, this code doesn’t usually come with noticeable symptoms or symptoms that affects the vehicle’s drivability. There is usually a decrease in fuel efficiency, but aside from the activation of the Check Engine light, that’s pretty much it.

Possible Causes

There are many factors that leads to this error code, some of the most common possible causes are:

  • Faulty EVAP pressure sensor
  • Missing or loose fuel cap
  • Broken, burned, or collapsed relief valve
  • Clogged fuel tank pressure relief valve
  • Broken or cracked charcoal canister
  • Damaged or defective fuel tank
  • Defective fuel pressure sensor, computer or wiring

How to Check

As with other error codes, mechanics retrieve the code from of the vehicle and take note of the freeze frame information to get a baseline for testing and verifying any repairs.

They observe the data stream through the scan tool and pay close attention to the readings in the fuel tank pressure. If the fuel pressure sensor is not working properly, the system will automatically think that there is no vacuum being generated, as the sensor can’t read the vacuum that is being created. The sensor will then send this information to the PCM, which relies for the leak test data.

Check and test the fuel pressure sensor wiring. Make sure there is 5V reference signal coming from the PCM, good ground, and a signature signal return circuit to the PCM.

While observing the data stream change (or lack thereof) with the scan tool, test the pressure sensor with a vacuum gauge while being connected to the wiring harness.

If the entire test above yields to the same result that is within the specified specs, then that would suggest that the problem is in the PCM itself.

How to Fix

Error Code P0451 is very straightforward error code. To get the proper access, the fuel tank must be removed. Then loose, broken, or damaged wires and electrical connectors must be repaired.

Also, check the lines for any cracks or holes. If everything seems to be okay after a visual inspection, then simply replacing the sensor can resolve the problem.

If evaporative pressure sensor looks good, then simply unplug all related controllers, and retest individual circuits using the DVOM. Replace open or shorted circuits as needed.

In many cases, the components of EVAP emission control system are needlessly replaced when there is a vacuum leak present, or the fuel cap is simply not properly tightened. Thus, it is important to conduct a thorough diagnosis to make sure you don’t waste money on replacing fully-functional components.