OBD2 codes.

Turbocharger/Supercharger "A" Underboost Condition

The turbocharger compresses the air going into the engine to give it more power; the engine computer regulates that boost pressure with the wastegate or the turbo's vanes. This code means the actual pressure stays clearly below what the engine computer asks for.

What the car has detected

When power was demanded for several seconds, the measured boost pressure stayed below the target by more than allowed.

You can drive carefully, but the car lacks power and may go into limp mode. If there is also blue smoke or the turbo makes noises, stop and have it checked: it could be damaged.

Symptoms

• Engine warning light on
• Lack of power, especially when accelerating
• Limp mode with limited power
• Whistling or hissing from a pressurised air leak
• Higher fuel consumption

Possible causes

• Boost leaks: split hoses, loose clamps or a leaking intercooler
• Seized wastegate or wastegate linkage
• Faulty turbo control valve or its vacuum lines
• Blocked intake or exhaust, for example a clogged catalytic converter or particulate filter
• Boost pressure sensor reading wrong
• Worn or damaged turbocharger

Checks for the garage

• Read the freeze frame data and compare actual with requested boost
• With the engine off and ignition on, check the boost sensor reads the same as atmospheric pressure
• Check the whole intake path, intercooler, hoses, clamps and vacuum lines
• Activate the turbo control valve with diagnostics and see whether the wastegate really moves
• Check that the exhaust is not blocked
• Check the turbo and its oil supply last

Next step

Check that the turbo and intercooler hoses are not loose or split, and have the wastegate and pressure sensor checked.

Go to a garage if you can't see an obvious leak or if the car goes into limp mode. If the turbo is noisy or there is blue smoke, don't push it and go as soon as possible.

Original sources · Date indicated: 2026-10-12