P3491 code meaning
Cylinder 12 Deactivation/Intake Valve Control Circuit Low
Powertrain system Generic code P34xx codes Check engine light
What P3491 means
A ground fault on the control line of the cylinder 12 deactivation or intake valve actuator produces P3491. No manufacturer documents this code publicly; J2012 reserves the codes for cylinders 9 to 12 for engines with more than eight cylinders. The Aventador's cylinder deactivation system, introduced in 2012, shuts down one bank of six below 135 km/h, and Bentley's W12 drops one bank; neither publishes cylinder-level diagnostic codes. On GM V8s the corresponding low test watches for under 0.5 ohm to ground while the output is switched off, every 250 ms, and needs 20 hits in 25 samples. Because a grounded low-side line powers the actuator, the cylinder can be shut off against the module's will. P3489 (open) and P3492 (high) are the other circuit faults, P3495 the exhaust low.
P3491 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P3491
- Check engine lamp on
- Cylinder shut-off blocked
- Possible misfire on cylinder 12
Common causes of P3491
- Most likelyCylinder 12 control wire shorted to ground
- PossibleInternal short in the cylinder 12 actuator
- Less likelyWater ingress at the connector
- Less likelyFaulty control module driver
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Cylinder deactivation solenoidWiring harnessECM
What each part of P3491 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 3 | Mixed range: generic and manufacturer-specific codes. |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 91 | The exact fault in that area. |
What to do next
- Note P3491 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P3491
What does P3491 mean?
P3491 means "Cylinder 12 Deactivation/Intake Valve Control Circuit Low". A ground fault on the control line of the cylinder 12 deactivation or intake valve actuator produces P3491. It is a generic code, so it means the same on every make.
Is it safe to drive with P3491?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P3491 fail an emissions test?
Yes, it can. P3491 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P3491 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P3491 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.