A failed EVAP vent valve that is stuck open (RepairPal; Honda Pilot/Passport manual, which lists this as the only possible cause). Damaged, corroded or loose wiring to the vent valve (RepairPal). A failed engine computer, which is rare (RepairPal). On 2023 Volkswagen Taos and Jetta 1.5T models in the bulletin's VIN ranges, engine computer software that needs an update (VW bulletin 01-23-08).

Replace the EVAP vent valve (RepairPal, Honda Pilot/Passport manual). Repair damaged wiring or connectors (RepairPal). Update the engine computer software where a bulletin calls for it, such as VW bulletin 01-23-08. Repair or replace the engine computer only after everything else checks out (RepairPal).

P2421🔧 Evaporative emissions (EVAP canister vent valve)Drive with caution, fix soon

P2421: EVAP Vent Valve Stuck Open (Meaning, Causes, Fixes)

Q: What does P2421 mean?

P2421 means the engine computer thinks the EVAP vent valve, which lets fresh air into the fuel vapor system, is stuck open and won't seal the system when told to close. RepairPal lists a failed vent valve, wiring problems and the engine computer as the common causes. Honda's service manual lists only the vent valve itself, and on some 2023 Volkswagens a software update comes first. The EVAP system stores fuel vapor from the tank in a charcoal canister and later draws it into the engine to be burned. The vent valve (Honda calls it the EVAP canister vent shut valve) is normally open so fresh air can enter, and the engine computer closes it to seal the system. Honda's Pilot and Passport service manual explains that because the valve is normally open, the computer can only catch it stuck open when it commands the valve closed during purging. With the valve closed, the fuel tank pressure sensor should show a small vacuum. If the reading drops by less than about 0.19 kPa (0.05 inHg), the computer decides the valve didn't close and stores P2421. Honda's test runs only once per drive cycle and only after the car has sat for at least 6 hours, with outside air at 40 F or warmer and the car driven at 25 mph or more. The first failure stores a pending code, and the light comes on only if it fails again on the next drive cycle. RepairPal says the same thing in simpler terms: the code sets when the computer sees the system venting when it should not.

Q: Can I keep driving with P2421?

Drive with caution, fix soon. Not an emergency, but get it diagnosed within a reasonable time. RepairPal rates P2421 as Moderate. The vent valve is part of the fuel vapor (EVAP) system, so the car usually keeps running, but the check engine light will keep you from passing an emissions inspection in most areas.

Reviewed 2026-10-24 · Based on 3 cited source(s)

What P2421 means

The EVAP system stores fuel vapor from the tank in a charcoal canister and later draws it into the engine to be burned. The vent valve (Honda calls it the EVAP canister vent shut valve) is normally open so fresh air can enter, and the engine computer closes it to seal the system. Honda's Pilot and Passport service manual explains that because the valve is normally open, the computer can only catch it stuck open when it commands the valve closed during purging. With the valve closed, the fuel tank pressure sensor should show a small vacuum. If the reading drops by less than about 0.19 kPa (0.05 inHg), the computer decides the valve didn't close and stores P2421. Honda's test runs only once per drive cycle and only after the car has sat for at least 6 hours, with outside air at 40 F or warmer and the car driven at 25 mph or more. The first failure stores a pending code, and the light comes on only if it fails again on the next drive cycle. RepairPal says the same thing in simpler terms: the code sets when the computer sees the system venting when it should not.

Severity: can I keep driving?

Drive with caution, fix soon

Not an emergency, but get it diagnosed within a reasonable time. RepairPal rates P2421 as Moderate. The vent valve is part of the fuel vapor (EVAP) system, so the car usually keeps running, but the check engine light will keep you from passing an emissions inspection in most areas.

Common symptoms

Check engine light on
Usually no drivability symptoms, because the vent valve is part of the fuel vapor system
Failed emissions or state inspection while the light is on
The light may take a couple of days to come back after clearing, since Honda's test needs a 6-hour soak and two drive cycles

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

A failed EVAP vent valve that is stuck open (RepairPal; Honda Pilot/Passport manual, which lists this as the only possible cause)
Damaged, corroded or loose wiring to the vent valve (RepairPal)
A failed engine computer, which is rare (RepairPal)
On 2023 Volkswagen Taos and Jetta 1.5T models in the bulletin's VIN ranges, engine computer software that needs an update (VW bulletin 01-23-08)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and freeze frame data. Fix any fuel tank pressure sensor codes first, since the test depends on that sensor reading correctly.
Check for technical service bulletins. Volkswagen's bulletin 01-23-08 says not to diagnose or replace parts for P2421 on affected 2023 Taos and Jetta models until the engine computer software is updated.
Find the charcoal canister and vent valve (usually near the fuel tank) and inspect the valve's connector and wiring for damage, corrosion and looseness.
A scan tool that can command the vent valve closed lets you hear it click and watch fuel tank pressure. Honda's troubleshooting uses a vacuum pump and gauge on the valve, and warns not to go beyond the specified vacuum or pressure because that can damage EVAP parts.
After a repair, clear the code and drive normally. Because the test needs the car to sit for hours first, confirm the light stays off over a few days of driving.

Typical fixes

Replace the EVAP vent valve (RepairPal, Honda Pilot/Passport manual)
Repair damaged wiring or connectors (RepairPal)
Update the engine computer software where a bulletin calls for it, such as VW bulletin 01-23-08
Repair or replace the engine computer only after everything else checks out (RepairPal)

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

You don't have a scan tool that can command the vent valve or read fuel tank pressure
The canister and vent valve are in a hard-to-reach spot near the tank
Your vehicle is covered by a software bulletin, which needs dealer equipment
You need the light off for an inspection deadline

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P2421 serious?

Not usually for driving. RepairPal rates it Moderate. The car typically runs normally, but the EVAP system can't seal itself to check for leaks, and the check engine light can fail an emissions inspection, so get it diagnosed soon.

What's the difference between P2421 and P2422?

P2421 means the vent valve is stuck open and P2422 means it's stuck closed. Honda detects stuck open when the fuel tank pressure barely changes after the valve is told to close, and stuck closed when there's too much vacuum after purging starts.

Why does P2421 come back days after I clear it?

On Hondas, the test runs only after the car has sat for at least 6 hours and then been driven at 25 mph or more, and the light needs two failed drive cycles. So it can take a couple of days of normal driving to see whether a fix worked.

Do the sources agree on the cause?

Not completely. Honda's manual lists only the vent valve stuck open, while RepairPal also lists wiring and the engine computer. Volkswagen's bulletin points to software on certain 2023 models before any parts are replaced.

How much does it cost to fix P2421?

None of these sources give a repair cost. It depends on whether it's a vent valve, wiring or a software update, so get a written estimate after diagnosis.

Sources

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.