|Trouble Code||Fault Location||Probable Cause|
|P2421||Evaporative emission (EVAP) vent valve - valve stuck open||EVAP vent valve|
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What Does Code P2421 Mean?
OBD II fault code P2421 is defined as “EVAP Vent Valve Stuck Open”, and is set when the PCM (Powertrain Control Module) detects that a part of the evaporative emission control system is not functioning correctly. The evaporative emission control system consists of the fuel tank, fuel lines, fuel vapor lines/hoses, vacuum lines, fuel filler cap, purge valve, charcoal canister, system pressure/flow sensors, vent valve control solenoid, as well as electrical wiring, connectors, and a dedicated leak detection pump on some models.
Flow or pressure sensors convert the varying pressure in the EVAP system to varying degrees of voltage; a code will be set and the CHECK ENGINE light illuminated if the signal voltage does not fall within the manufacturer’s specifications. Note that on some vehicles, the code will only be stored after several failure cycles, while on others the code will be set after a single failure. In some cases where a code is present in the absence of an illuminated warning light, the code may be read as a “pending code”.
The function of the EVAP system is to collect, and route fuel vapors to the engine to be burned as part of the regular air/fuel mixture, before the vapors can escape into the atmosphere. In most applications, fuel vapors are allowed to accumulate until a set pressure is reached. Upon this happening, the PCM receives a signal from a pressure sensor to command a purge valve to open, through which valve engine vacuum sucks the vapors into the engine to be combusted.
Be aware that on some older systems the fuel vapors are not delivered to the inlet tract to be burned; instead, the vapors are passed through a charcoal canister, where the vapors are absorbed by the activated charcoal. When the charcoal canister becomes clogged, damaged, or unserviceable for any of a number of reasons, the pressure in the EVAP system can rise to the point where a pressure sensor will alert the PCM to set a code, and possibly illuminate the CHECK ENGINE warning light.
Note that on some applications, a dedicated air pump is used to pressurize the EVAP system at set intervals as a means of detecting leaks in the system. In these designs, compressed air takes the place of fuel vapors, but the principle remains the same- the system should contain the compressed air without any significant pressure drops occurring. Note that on these designs, failure of the pressure pump can also set code P2421.
The image below shows a simplified schematic of a typical EVAP system that routes fuel vapors to the inlet tract. In these designs, the fuel vapors are burn along with the air/fuel mixture.
What are the common causes of code P2421?
Leading causes of code P2421 are defective leak detection pumps, and defective vent/purge valves, or defective control solenoids. Other possible causes include the following-
Shorted, burnt, corroded, or otherwise damaged wiring and/or connectors
Split, cracked, or otherwise damaged vacuum and/or fuel vapor lines
Worn or damaged seals on fuel filler caps
Incorrectly installed fuel filler caps
Clogged, corroded, or otherwise damaged charcoal canisters
PCM failure is a rare event, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P2421?
In most cases where code P2421 is present, there may not be any discernable symptoms present, other than the stored code and an illuminated warning light. Note however that depending on the design of the EVAP system, there may be other evaporative emission control system codes present along with P2421.
How do you troubleshoot code P2421?
NOTE #1: A repair manual and a smoke machine may be required to diagnose code P2421 accurately. Vacuum leaks are sometimes all but invisible, which makes a smoke machine a useful tool with which to pinpoint small leaks in the vacuum system. However, testing an EVAP system with smoke is best done with the vehicle on a hoist to make inspecting fuel and vacuum lines easier.
NOTE#2: On applications where an air pump is present, it is generally possible to activate the pump with a scanner for the purpose of testing the EVAP system. Failure of the pump is a leading cause of code P2421; consult the manual on the testing procedure of the pump as a first step in the diagnostic/repair procedure to avoid spending inordinate amounts of time looking for vacuum leaks.
Record all codes and available freeze frame data; this information can be useful if an intermittent fault is diagnosed later on.
Refer to Note#2 if the vehicle is fitted with an air pump. If the scanner confirms that the pump is not working, consult the manual to determine the color-coding, location, routing, and function of each wire in the pump control circuit. Perform a thorough visual inspection on the wiring, and look for damaged, burnt, corroded, or otherwise damage wiring and connectors. Repair all wiring and/or connectors as required.
If no visible damage to the wiring is found, perform resistance, continuity, ground, and reference voltage checks on the wiring. If all readings fall within the manufacturer’s specifications, the pump is likely faulty. Consult the manual on the recommended procedure to test the pump, and replace it if it does not comply with stated resistance and continuity values.
NOTE: Be sure to disconnect the air pump from the PCM before starting continuity and resistance checks on the wiring to avoid damage to the controller.
If repairs to wiring had been made, or if the pump had been replaced, clear the code, and operate the vehicle to see if the code returns. If it does return, consult the manual on the location, color-coding, routing, location, and function of each wire in the vent valve control solenoid and circuit.
If possible, activate the vent valve control solenoid with the scanner. If the solenoid cannot be activated with a scanner, perform a visual inspection of associated wiring. Look for damaged, shorted, burnt, or otherwise damaged wiring and connectors. Repair as required.
If no visible damage is found, perform reference voltage, resistance, ground, and continuity checks on the associated wiring, but be sure to check the internal resistance of the control solenoid as well. Consult the manual on the exact electrical values, and repair or replace wiring (or the control solenoid if required) to make sure all values fall within the manufacturer’s specifications.
NOTE: Many designs use both a vent valve and a purge valve. Be sure to check both circuits (including the control solenoids) for resistance, continuity, ground, and reference voltage. Compare all obtained readings to the values stated in the manual, and replace solenoids that do not conform to manufacturer’s specifications.
TIP: At this point it is a good idea to check the charcoal canister as well. Remove the canister, and shake it to listen for loose particles; replace the canister if a rattling sound is heard. Also check for the presence of corrosion anywhere on the canister. Corrosion is a sure sign that the canister is clogged or otherwise unserviceable. Replace the canister if there is any doubt about its condition.
After the repairs outlined in Step 3 had been made, clear the code, and operate the vehicle to see if the code returns. If it does, it likely that there is either a vacuum leak present, or that the fuel filler cap is not gas tight.
Consult the manual to determine the routing of fuel and vacuum lines. Check for damaged, kinked, perforated, or otherwise damaged vacuum lines. Note that on some applications, it may be necessary to remove protective covers and shield from the underside of the vehicle to expose fuel and vacuum lines. This check is best performed on a vehicle hoist.
If the fuel vapors are routed into the inlet tract, pay particular attention to the condition of the connection. Even if the hose is not visibly cracked or split at this point, time and heat could have hardened it to the point where it no longer forms a perfect seal. Repair this connection if the hose is not soft and pliable, or if there is any doubt about the quality of the host at the connection point.
NOTE: Resist the temptation to repair vacuum lines. The better option is always to replace leaking vacuum lines with OEM parts to avoid leaks in poorly executed repairs.
If no visible damage to vacuum lines is found, disconnect the line at the point where it connects to the inlet manifold, or the charcoal canister, and connect the outlet of the smoke machine to the vacuum line.
If possible, command the vent valve control solenoid open with a scanner, or apply direct current to it to keep it open during this test, since the entire EVAP system needs to be filled with smoke to pinpoint vacuum leaks that might not be immediately visible.
NOTE: Be sure to follow the instructions in the manual if direct current is to be applied to the control solenoid to avoid accidental short circuits, and/or current overloads that could damage the vehicle’s electrical system
Allow the system to fill with smoke; pay particular attention to the area around the fuel filler cap. This seal should by 100% gas tight, and escaping smoke is a sure sign that fuel vapors are escaping past the filler cap seal. Replace the filler cap if any smoke is escaping past it, no matter how small the amount of smoke.
If the fuel filer cap proves to be gas tight, the smoke in the EVAP system will escape through all and any other leak paths, but be aware that it might take several minutes for small leaks to become visible. Do NOT assume that there are no leaks if none show up in the first few seconds; some leaks are sometimes so small that even with smoke, it might take some minutes before they become apparent.
Repair all leaks found by replacing the vacuum line with an OEM part. However, to be sure that all connections and joints are leak proof after the repair/replacement, retest the system with smoke to verify that there are no leaks. If no leaks are found, clear the code, and operate the vehicle to see if the code returns.
If all electrical values fall within specifications, there are no vacuum leaks, and one or more proven defective components have been replaced, it is unlikely that the code will return. If it does return, it is likely that there may be an intermittent fault present. Intermittent faults can sometimes be very difficult to find and repair, and in extreme cases it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.
NOTE: Bear in mind that on some applications, several drive cycles may be required before the code will manifest again, so do not assume that the repair had been successful until at least six or so drive cycles had been completed.
Codes Related to P2421
P2422 – Relates to Evaporative Emission System Vent Shut
Note that while P2421 is a generic code on many, if not most applications, its presence on some vehicles may not indicate a problem specifically related to the EVAP system vent valve. For instance, on some GM products this code refers to swapped oxygen sensor signals, while on some Volvo products this code refers specifically to the leak detection pump control module. It is therefore recommended that the exact definition of this code be researched as it relates to the application being worked on.