P0391 code meaning
Camshaft Position Sensor B Circuit Range/Performance Bank 2
Powertrain system Generic code P03xx codes Check engine light
What P0391 means
Camshaft position sensor B is the exhaust camshaft sensor; bank 2 is the side without cylinder 1, so the code only occurs on V and boxer engines with one exhaust cam sensor per bank. P0391 means the bank 2 exhaust cam signal is present, but the number or position of its pulses does not fit the crankshaft. GM's 3.0 twin-turbo V6 (LGW) expects 4 to 6 pulses in the first 12 reference events and 398 to 402 per 100 engine cycles on the four-tooth wheel, after two trips; the Saab 9-4X 2.8 turbo V6 (LAU) checks pulses per crank revolution below 2,520 rpm. The 2004 Northstar requires each edge within a window of crank teeth, and GM's 2004 3.6-litre V6 (LY7) counts implausible signals over two drive cycles. The phaser then goes to its base position and the engine cranks longer. Mechanical causes – damaged tone wheel, jumped chain – matter more here than with P0390.
P0391 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 8 hours
- Clears after repairAfter a drive cycle
Symptoms of P0391
- Check engine light on
- Loss of power (bank 2 exhaust cam timing in its base position)
- Longer cranking
- Rough idle
- Occasional stalling
Common causes of P0391
- Most likelyTone wheel on the bank 2 exhaust phaser damaged, loose or wrongly fitted
- PossibleInterference or loose contact producing extra edges
- PossibleSensor loose, wrong O-ring or gap, chips on the magnet
- PossibleTiming chain stretched or jumped
- PossibleSensor faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Camshaft position sensor exhaustCamshaft tone wheelSensor wiringEngine control moduleCam phaser exhaust bank2Timing chain
What each part of P0391 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 3 | Area: ignition system or misfire. |
| 91 | The exact fault in that area. |
What to do next
- Note P0391 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 P0391
What does P0391 mean?
P0391 means "Camshaft Position Sensor B Circuit Range/Performance Bank 2". Camshaft position sensor B is the exhaust camshaft sensor; bank 2 is the side without cylinder 1, so the code only occurs on V and boxer engines with one exhaust cam sensor per bank. It is a generic code, so it means the same on every make.
Is it safe to drive with P0391?
Drive gently and only to a workshop. With P0391 the car can go into limp mode (reduced power) to protect itself.
Will P0391 fail an emissions test?
Yes, it can. P0391 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 P0391 to fix?
Difficulty: medium. A typical repair takes 0.5 to 8 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0391 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.