P3420Cylinder 3 Deactivation/Intake Valve Control Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- P3420 reports that the cylinder 3 deactivation control line carries supply voltage where the module expects to pull it low. On GM's L84 the test runs only while the low-side output is switched on: a connection to battery voltage below 0.5 ohm, confirmed in 20 out of 25 samples, sets a Type B code. According to NHTSA's 2025 investigation into GM's DFM V8s, every cylinder has exactly one lifter oil solenoid, run by a peak-and-hold driver, so a single short disables the whole cylinder's switching. The Gen IV controller also has a separate high-side stage for this solenoid with its own codes, which helps tell which half of the circuit failed. P3419 covers the short to ground. The engine just keeps all eight cylinders working.
- What you may notice
- Check engine lamp on
- Fuel economy mode unavailable
- Likely causes
- LikelyControl wire of cylinder 3 touching a powered conductor
- PossibleShorted turns in the cylinder 3 solenoid winding
- RareOutput transistor in the engine control module damaged
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.