What this code means
P2421 may indicate that the EVAP system pressure response is not matching what the PCM expects when it commands the vent side closed during purge operation.
What the vehicle may do
- The check engine light may be on.
- The vehicle may have little or no noticeable drivability change.
- EVAP-related symptoms can often be subtle.
Possible fault areas
- Possible EVAP vent hose blockage or restriction.
- Possible purge-line connection, sealing, or damage issue.
- Possible canister-area component or O-ring damage.
- Possible EVAP canister vent shut valve issue.
- Possible fuel fill pipe, vapor recirculation tube, shut-off valve, seal, or fuel tank issue.
Diagnostic path
Opening and code logic
On this 2016 to 2024 Honda Civic with the 1.5 turbo, P2421 is an EVAP system performance code. In plain terms, the PCM is looking for the fuel tank pressure signal to respond when it commands the EVAP canister vent shut valve closed during EVAP purging. If that pressure change is lower than expected, it can set this code. The driver may only notice a check engine light, and EVAP faults can often be subtle. The possible fault areas stay broad at first: venting, purge-line sealing, the canister area, the vent shut valve, the fuel fill and vapor recirculation path, the shut-off valve, and, only after the earlier checks pass, the fuel tank itself.
Test precautions and setup mindset
Before getting into the EVAP checks, start with the basic system checks and follow a structured diagnostic approach. This system is sensitive to vacuum and pressure, so do not go beyond the vacuum or pressure called out in the test. Too much vacuum or pressure can damage EVAP components or eventually damage the fuel tank. The best leak-detection condition is less than a full tank of fresh fuel. If possible, add 1 gallon of fresh fuel to the tank, as long as that will not fill the tank, right before starting these checks.
Confirm the fault
First, turn the vehicle to the ON mode. Then run the HDS function test for this EVAP fault. If the test result is not OK, the failure has been duplicated. That gives you a live failure path to work from instead of chasing an intermittent condition blindly.
Check the EVAP canister vent hose
Next, disconnect the EVAP canister vent hose and inspect it for a blockage or restriction. The expected result is no blockage and no restriction. If that check is abnormal, stay in that area; the problem is with the EVAP canister vent hose.
Visually inspect the purge-line side
After the vent hose check, inspect the EVAP canister purge line between the EVAP canister purge valve and the EVAP canister. Look for poor connections or damage. At the same time, check the EVAP canister, the FTP sensor, the EVAP canister vent shut valve, and the O-rings for damage. The expected condition is good connections and no damage. If this visual inspection is abnormal, the problem is in the parts on the EVAP canister purge line.
Vacuum-test the EVAP canister purge line
Now test the purge line sealing. Disconnect the EVAP canister vent hose and the fuel vent tube from the EVAP canister, then plug each port. In the engine compartment, disconnect the vacuum hose, or purge line, from the EVAP canister purge valve. Connect a vacuum pump and gauge, 0-30 inHg, to that hose. Turn the vehicle to the ON mode. Apply vacuum only until the HDS FTP Sensor Voltage reads 1.90 V (-0.59 inHg, -15.1 mmHg). Be careful not to exceed the vacuum, because the FTP sensor can be damaged. Hold the reading for 1 minute. A normal result is that the HDS FTP Sensor Voltage does not increases more than 0.2 V (0.1 inHg, 2.5 mmHg). If this test is abnormal, the problem is with the parts on the EVAP canister purge line.
Test the EVAP canister vent shut valve
Move to the EVAP canister vent shut valve only after the EVAP canister purge line tests normal. Disconnect the fuel vent tube from the EVAP canister and plug the port. Then disconnect the vacuum hose, or purge line, from the EVAP canister purge valve in the engine compartment, and connect the vacuum pump and gauge, 0-30 inHg, to that hose. Turn the vehicle to the ON mode and command HDS CVS ON. Apply vacuum only until the HDS FTP Sensor Voltage reads 1.90 V (-0.59 inHg, -15.1 mmHg). Again, do not exceed the vacuum, because the FTP sensor can be damaged. Confirm the FTP Sensor Voltage for 1 minute. A normal result is that the HDS FTP Sensor Voltage does not increases more than 0.2 V (0.1 inHg, 2.5 mmHg). If this test is abnormal, the problem is with the EVAP canister vent shut valve.
Inspect the fuel fill and vapor recirculation path
If the earlier EVAP checks pass, inspect the fuel fill pipe and the fuel tank vapor recirculation tube. Check for poor connections or damage. The expected result is that the connections are OK and there is no damage. If this inspection is abnormal, the problem is with the fuel fill pipe and/or the fuel tank vapor recirculation tube.
Inspect the shut-off valve and seal
Next, with the mis-fuel inhibit device removed, visually inspect that the shut-off valve is closed. If it is not closed, the problem is with the main flap unit. Then, again with the mis-fuel inhibit device removed, inspect the flap seal of the shut-off valve and the mating surface for a missing or damaged seal. The expected result is no missing or damaged seals. If that seal inspection is abnormal, the problem is with the main flap unit.
Final diagnostic call and verification
The fuel tank is the final call in this path. If the other inspection items all indicate a normal condition, then the remaining fuel tank is determined to be the cause. Keep verification separate from the individual tests: after the fault found in the diagnostic path is corrected, rerun the confirmation routine and make sure the failure no longer duplicates. The takeaway is to diagnose P2421 by proving the commanded EVAP pressure response step by step, and only move deeper after the previous section tests normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
P2421 is best approached as a command-versus-response EVAP diagnosis: confirm the fault, protect the system from excessive vacuum or pressure, and work through each sealing and venting check in order.
For more guided automotive diagnostics, visit STEP Diagnostics.
