P2391 compares where the B intake throttle should be with where its position sensor says it is. If the gap stays too large under a steady command, the module concludes that the flap is sluggish or obstructed, typically by soot and oil deposits from EGR and crankcase gases, a worn gear stage or a weak motor. No published manufacturer table covers P2391. GM's 2025 diesels judge the A valve (P02E1) on a tracking error above 10 % in 960 of 1,200 samples at 6.25 ms, only once coolant has reached 40 °C and outside the cold-start strategy, and light the MIL in the first trip. Unlike the stuck codes P2394 and P2395, this one does not say in which direction the flap lags. Expect higher NOx, possibly blocked regeneration and sometimes reduced power.
Medium severity. Can I keep driving?You can drive in moderation. Get it repaired soon, because a loose flap can cause major damage on some engines.
Symptoms
- MIL on
- Regeneration of the particulate filter does not complete
- Occasionally reduced power or uneven response
Most likely causes
- Soot and oil deposits making the B flap stiff
- Worn plastic gears or shaft bearing in the throttle unit
- Weak motor or high resistance in the drive circuit
- Position sensor reporting an offset angle
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P2391 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- 91: Specific fault in that system
Frequently asked questions
Can I keep driving with code P2391?
You can drive in moderation. Get it repaired soon, because a loose flap can cause major damage on some engines.
What causes code P2391?
The most common causes are: soot and oil deposits making the B flap stiff; worn plastic gears or shaft bearing in the throttle unit; weak motor or high resistance in the drive circuit.
Related codes
- P2394Diesel Intake Air Flow B Control Stuck Open
- P2395Diesel Intake Air Flow B Control Stuck Closed
- P2397Diesel Intake Air Flow B Position Sensor Range/Performance
- P239BDiesel Intake Air Flow B Control Motor Current Range/Performance
- P02E1Diesel Intake Air Flow A Control Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 25OBDG04B
- ms-motorservice.com: throttle valves and regulating throttles
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.