# P0451: Evaporative Emission System Pressure Sensor/Switch "A" Circuit Range/Performance **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$1,100 - **Urgency:** Keep driving - **Most likely:** Failing EVAP (tank) pressure sensor - **DIY?:** Steps 1-2 ## Quick answer P0451 means the engine computer is getting readings from the EVAP pressure sensor that do not make sense for the conditions. This sensor, often called the fuel tank pressure sensor, measures pressure and vacuum in the fuel vapor system. A failing sensor is the most common cause, but wiring, a blocked sensor passage, or liquid fuel in the vapor lines can also set it. ## What does P0451 mean? The EVAP pressure sensor watches the pressure inside the sealed fuel vapor system so the engine computer can run its leak tests. Depending on the vehicle, it sits on top of the fuel tank, as part of the fuel pump module, or on or near the charcoal canister. Most are three-wire sensors that receive a 5-volt reference and send back a signal voltage that rises or falls with pressure. Some older systems use an on-off pressure switch instead, which is why the code says sensor or switch. Range/performance means the signal is within electrical limits but behaves wrongly. The computer may expect a reading near zero with the system open to air, a steady change when the purge valve pulls vacuum, or a stable reading when the system is sealed. If the reading is stuck, jumps around, or disagrees with what was commanded, it sets P0451, usually after two trips. "A" names the first or only sensor of this kind. ## What are the symptoms of P0451? - Check engine light on with normal driving - No other symptoms at all on many vehicles - EVAP monitor not ready, or a failed emissions test - Other EVAP leak codes that may turn out to be false - Fuel smell or slow refueling only if a vapor line is blocked or flooded ## What causes P0451? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failing EVAP pressure sensor** (most common). The sensor's tiny diaphragm and electronics live on or near the fuel tank, where they face fuel vapor, exhaust heat, and road spray. With age, the output can drift, stick, or turn noisy. A key sign is a reading that is not close to zero pressure with the gas cap off and the engine off. 2. **Wiring or connector problems** (common). The sensor harness usually runs under the car to the tank or canister, exposed to water, salt, and debris. Corroded pins or a chafed wire can make the signal flicker or drift without dropping out completely. This is common on older vehicles in snowy states, and moving the harness while watching live data often reveals it. 3. **Blocked or kinked sensor passage or hose** (less common). On designs where the sensor connects to the vapor system through a small hose or port, a kink, a crushed line, or debris can trap pressure at the sensor. The reading then stays put while the rest of the system changes. Recent work on the tank or fuel pump is a common trigger for this. 4. **Liquid fuel in the vapor lines** (less common). Repeatedly topping off the tank after the pump clicks off can push liquid gasoline into vapor lines and the canister. Liquid in the sensor passage can upset its readings and sometimes damages the canister too. Slow refueling or a fuel smell after filling up, alongside P0451, points this way. 5. **Purge or vent valve behaving wrongly** (rare). A purge valve or vent valve that sticks can produce pressure the computer did not expect, which some systems flag as a sensor performance fault. This is uncommon, but a shop should check both valves with a scan tool before replacing an expensive tank-mounted sensor or a fuel pump module. ## How do you diagnose P0451? 1. Check for other EVAP codes and any 5-volt reference codes that might share the same circuit. 2. With the engine off, remove the gas cap and check that the sensor reads close to zero pressure on a scan tool. 3. Watch the reading during a commanded purge or leak test and compare it with the sensor's specification. 4. Inspect the harness and connector at the tank or canister for corrosion, damage, and water. 5. Check the sensor's reference, ground, and signal voltages at the connector against the service manual. ## How do you fix P0451, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace EVAP pressure sensor | $150-$450 | Higher when the tank must be lowered to reach it. | | Repair wiring or connector | $100-$350 | Common fix on older vehicles in road-salt states. | | Replace fuel pump module with built-in sensor | $500-$1,100 | Only on vehicles where the sensor is not sold separately. | Full price breakdowns: [fuel pump replacement cost](/cost/fuel-pump-replacement). ## Is it safe to drive with P0451? > **Safe to drive.** Yes, P0451 is safe to drive with, because it does not affect how the engine runs. The main problems are a check engine light that can hide other faults and a likely failed emissions test. Have any strong fuel smell checked promptly. ## Will P0451 fail an emissions test? P0451 fails an OBD-based emissions test in most states while the check engine light is on. Because the EVAP leak tests depend on this sensor, the EVAP monitor usually cannot complete until it is fixed. Some states allow one incomplete monitor on newer cars, but the stored code itself still causes a fail. ## After the repair: clearing P0451 After replacing the sensor or fixing the wiring, clear the code and check that the reading sits near zero with the cap off. Then drive normally for several days, including overnight cool-downs and, on many cars, a fuel level between about a quarter and three-quarters of a tank. A ready EVAP monitor confirms the fix. ## How to prevent P0451 - Stop fueling at the first automatic click, which keeps liquid gasoline out of vapor lines. - Rinse road salt from the underbody in winter to slow connector and wiring corrosion. - Ask for a check of EVAP lines and connectors after any fuel tank or pump work. - Use dielectric grease on the sensor connector whenever it is unplugged for service. ## Codes often seen with P0451 - [P0452](/codes/p0452): Evaporative Emission System Pressure Sensor/Switch "A" Circuit Low - [P0455](/codes/p0455): Evaporative Emission System Leak Detected (large leak) - [P0442](/codes/p0442): Evaporative Emission System Leak Detected (small leak) - [P0456](/codes/p0456): Evaporative Emission System Leak Detected (very small leak) - [P0440](/codes/p0440): Evaporative Emission System - [P0446](/codes/p0446): Evaporative Emission System Vent Control Circuit ## Frequently asked questions ### Can I drive with a P0451 code? Yes, P0451 does not change how the car drives or harm the engine. It does keep the check engine light on, which can mask a new problem, and it usually means a failed emissions test. Plan a repair before your next inspection, and sooner if you smell fuel. ### What is an EVAP pressure sensor? It is a small sensor that measures pressure and vacuum in the fuel vapor system, often called the fuel tank pressure sensor. The engine computer uses it to check the system for leaks. It usually sits on top of the fuel tank, on the fuel pump module, or near the charcoal canister. ### How much does it cost to fix P0451? Most P0451 repairs cost $100 to $1,100 at an independent shop. A separate sensor typically runs $150 to $450, and a wiring repair $100 to $350. If the sensor is built into the fuel pump module and not sold separately, the module can cost $500 to $1,100 installed. ### What is the difference between P0451 and P0452? P0451 means the sensor's signal is within its electrical range but behaves wrongly, such as stuck or erratic readings. P0452 means the signal voltage is too low, which usually points to a short to ground, a lost reference voltage, or a failed sensor. The testing steps overlap. ### Can a bad gas cap cause P0451? Not usually. A bad gas cap typically sets leak codes such as P0455 or P0457 rather than a sensor performance code. However, on some vehicles a system that will not seal can confuse the sensor test, so check the cap and look at all stored EVAP codes together. ### Can I replace the EVAP pressure sensor myself? Sometimes. If the sensor sits on the canister or is reachable on top of the tank, it is a simple unplug-and-unclip job. Many tank-top sensors require lowering the tank, which means handling fuel and is safer at a shop. Check where yours is mounted before buying parts. ### How do I test a fuel tank pressure sensor? With the engine off, remove the gas cap and read the sensor on a scan tool, where it should show close to zero pressure. A shop then checks the 5-volt reference, ground, and signal voltage at the connector. Readings that drift, jump, or never change point to a faulty sensor. ### Will P0451 make me fail an emissions test? Yes, in most states P0451 fails an OBD-based test while the check engine light is on. The EVAP monitor also relies on this sensor, so it may not finish until the repair is done. After fixing it, allow several days of normal driving before your test. ### Can overfilling the gas tank cause P0451? It can contribute. Topping off after the pump clicks can force liquid fuel into vapor lines and the sensor passage, which can upset its readings. It can also damage the charcoal canister, which is costly to replace. Stop at the first click to protect the whole EVAP system. ### Why does P0451 come and go? An intermittent P0451 often comes from a corroded connector or a wire that loses contact over bumps or in wet weather. A sensor that is starting to fail can also drift only at certain temperatures. Note when the light appears, and ask the shop to check the harness under the car. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0440 Evaporative Emission System](https://diagxcar.com/codes/p0440) - [P0441 Evaporative Emission System Incorrect Purge Flow](https://diagxcar.com/codes/p0441) - [P0442 Evaporative Emission System Leak Detected (small leak)](https://diagxcar.com/codes/p0442) - [P0443 Evaporative Emission System Purge Control Valve "A" Circuit](https://diagxcar.com/codes/p0443) - [P0446 Evaporative Emission System Vent Control Circuit](https://diagxcar.com/codes/p0446) - [P0449 Evaporative Emission System Vent Valve/Solenoid Circuit](https://diagxcar.com/codes/p0449) - [P0452 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0452) - [P0455 Evaporative Emission System Leak Detected (large leak)](https://diagxcar.com/codes/p0455) - [P0456 Evaporative Emission System Leak Detected (very small leak)](https://diagxcar.com/codes/p0456) - [P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)](https://diagxcar.com/codes/p0457) ## Symptoms - [Why does my car smell like gas?](https://diagxcar.com/symptoms/car-smells-like-gas) ## Repair costs - [Fuel pump replacement cost](https://diagxcar.com/cost/fuel-pump-replacement): $450-$1,300 --- Source page: https://diagxcar.com/codes/p0451 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.