What does it mean?
P3420 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 3 Deactivation/Intake Valve Control Circuit High . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- MIL (check engine lamp) illuminated with the deactivation function locked out
- Fuel consumption rises since the engine can no longer switch to reduced-cylinder operation
- Rough idle or a shake if the cylinder 3 valve is stuck in one state
- Ticking or light knock from the cylinder head
- Misfire on cylinder 3 under light load
- Dull throttle response and occasional hesitation
Likely causes
- Short to battery voltage where the control wire runs alongside a fused feed
- Open or corroded ground return, leaving the driver output floating at supply voltage
- Open solenoid winding so the circuit is never loaded down
- Oil-contaminated or corroded terminals in the solenoid connector; this circuit is notorious for oil migration
- Water intrusion bridging a power pin to the control pin inside a connector
- Internal PCM driver failure, considered only after the wiring is proven good
How to fix it
- Note the freeze frame conditions, then try to reproduce the fault with the scan tool commanding the solenoid
- Inspect the connector for oil, corrosion, melted plastic and loose terminals
- Back-probe the control wire with the key on and compare it with a healthy solenoid on the same bank
- Disconnect the solenoid and the module, then test the control wire for a short to any powered circuit
- Measure the solenoid resistance; an open coil produces a high-side reading without a wiring fault
- Check the ground return path and terminal tension in the connector
- Repair the harness or renew the solenoid, then verify with the scan tool that the output switches cleanly