P3420 indicates that the intake valve control circuit for cylinder 3 is reporting a high voltage condition, meaning the valve is not deactivating as commanded. This is part of the cylinder deactivation system used to improve fuel efficiency.
The solenoid that actuates the intake valve may have failed or become stuck, causing an abnormal high voltage reading.
Chafed, corroded, or broken wires in the circuit can cause a high voltage condition to be sensed.
A poor connection at the valve control connector can create erratic voltage signals.
The valve itself may be seized or the actuator mechanism may be jammed, preventing proper deactivation.
Only after ruling out hardware faults should a malfunctioning control module be considered, which is rare for this code.
It is possible but uncommon. Most occurrences are due to the solenoid, wiring, or valve itself. Replacing the PCM should only be done after all other components have been verified as functional.
Clearing the code may temporarily turn off the Check Engine Light, but if the underlying fault remains, the code will return and the symptoms will persist.
The vehicle will operate in a reduced‑efficiency mode, and a stuck valve can cause rough running. It is advisable to address the issue promptly to avoid possible engine damage.
Not necessarily. Often the solenoid or wiring can be repaired or replaced without disturbing the valve itself. Only a mechanical seizure of the valve would require valve or actuator replacement.