P3491 indicates that the engine control module has detected a lower-than-expected voltage on the control circuit for the cylinder 12 intake valve deactivation solenoid. This typically means the circuit is shorted to ground, open, or the solenoid itself is internally faulty. As a result, cylinder deactivation for that cylinder is disabled and the check engine light illuminates.
The solenoid coil may be internally shorted or open, causing the control circuit voltage to drop below the commanded threshold and triggering the fault.
Damaged or chafed wiring between the PCM and the solenoid can allow the control wire to short against the engine block or another ground source, pulling voltage low.
A broken wire, pinched harness, or corroded terminal can prevent adequate voltage from reaching the solenoid, resulting in a circuit low reading.
Loose, bent, or moisture-intruded connector pins can introduce unwanted resistance or intermittently pull the control circuit voltage below the expected range.
Although uncommon, an internal fault in the engine control module's solenoid driver can fail to source or sink current properly, causing a persistent low circuit voltage.
Yes. The engine typically defaults to normal all-cylinder operation, so immediate drivability or safety issues are unlikely, though fuel economy may decrease.
No. P3491 is an electrical circuit fault for the deactivation solenoid. It does not imply damage to the intake valve, lifter, or piston, though oil contamination can affect both wiring and solenoids.
Not as a first step. PCM failure is a rare cause of P3491. Most cases are resolved by replacing the solenoid or repairing the harness. The PCM should only be replaced after exhaustive electrical testing proves the module is at fault.
No. This code is specific to cylinder 12. Replace only the affected solenoid or repair its individual circuit unless inspection finds widespread connector or wiring deterioration.