What does it mean?
This diagnostic code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II (Dodge, Ram, Ford, GMC, Chevrolet, VW, Audi, Toyota, etc.). Although generic, specific repair steps may vary by brand/model.
If your OBD-II vehicle has displayed a P0451 code, it simply means that the Powertrain Control Module (PCM) has detected a malfunction in the Evaporative Emission (EVAP) system pressure sensor circuit.
The EVAP system uses a vented reservoir (commonly called a canister) to store excess fuel vapors until the engine is operating under the right conditions to burn them completely. It is designed to capture fuel vapors before they escape into the atmosphere.
The charcoal canister, EVAP pressure sensor, purge valve/solenoid, vent control valve/solenoid, and a complex system of metal tubes and rubber hoses that run from the fuel tank to the engine compartment are all crucial to the operation of the EVAP system.
Fuel vapor pressure, created when fuel is stored, acts as the propellant and causes vapors to be evacuated through a network of metal tubes and rubber hoses; eventually reaching the canister. Vapors from the fuel storage tank are evacuated through the overpressure valve (at the top of the fuel tank) and forced through a complex network of metal and rubber lines to the charcoal canister, where they accumulate. The canister contains a charcoal element that absorbs the fuel vapors and holds them for release at the appropriate time.
The purge control valve/solenoid is responsible for regulating intake vacuum in the EVAP canister so that fuel vapors can be drawn into the engine when conditions are ideal for them to be burned instead of released into the atmosphere. The PCM electronically controls the purge control valve/solenoid, which is the heart of the EVAP system.
The EVAP system/fuel tank pressure is monitored by the PCM using the EVAP pressure sensor. Located in the fuel tank and integrated into the fuel pump/fuel level sender assembly, the EVAP pressure sensor can be quite difficult to access. If the PCM detects that the EVAP and/or fuel tank pressure is not within a preprogrammed range, over a defined period and under certain circumstances, a P0451 code will be stored and the Malfunction Indicator Lamp (MIL) may illuminate.
Evaporative emission-related trouble codes include P0450, P0452, P0453, P0454, P0455, P0456, P0457, P0458, and P0459.
Symptoms
Symptoms of this code may include:
In most cases, no symptoms will manifest with the P0451 code
A slight decrease in fuel efficiency
MIL (Malfunction Indicator Lamp) illumination
Causes
Causes
Possible causes for this code being set are:
Faulty EVAP pressure sensor
Loose or missing fuel cap
Clogged fuel tank overpressure valve
Cracked, collapsed, or burned EVAP hoses/lines
Cracked or broken charcoal canister
Diagnostic and Repair Procedures
When diagnosing a P0451 code, I will need a diagnostic scanner, a digital volt/ohmmeter, a reliable vehicle information source like All Data DIY, and possibly a smoke machine.
I like to start with a visual inspection of the EVAP system hoses and lines, as well as electrical wiring harnesses and connectors. I focus on components routed near sharp edges or hot exhaust components. This is also a good time to remove the fuel cap and inspect the seal. Be sure to tighten it properly.
Proceed by connecting the scanner to the vehicle’s diagnostic port and retrieving all stored codes and freeze frame data. I usually note this information as it can be very helpful later in my diagnosis. Clear the codes and test-drive the vehicle until it enters OBD-II readiness mode or the code resets. EVAP codes will almost always require multiple drive cycles before they reset.
I normally use the diagnostic data stream (from the scanner) to observe the EVAP pressure sensor signal. If the system pressure meets the manufacturer’s recommended specifications, I know I’ve fixed the issue by tightening or replacing the fuel cap.
This is a pressure sensor circuit code, so I would test the EVAP pressure sensor before performing a smoke test. Testing the EVAP pressure sensor can be aggravating because it is usually located at the top of the fuel storage tank. Access the sensor and follow the manufacturer’s recommendations for testing, then replace the sensor if it does not meet specifications.
If the EVAP pressure sensor appears to be good, disconnect all associated controllers and test individual circuits with the DVOM. Replace any open or shorted circuits as necessary.
Additional diagnostic notes:
Low or high EVAP pressure can lead to a P0451 being stored
This code can be due to electrical or mechanical malfunctions