Loss of power and limp mode: causes and affected cars

In limp mode, the engine computer deliberately limits power to protect the engine after detecting a fault. The car still moves, but without pulling power, and often with the engine warning light on. The cause can be the emissions system, the turbo, the injection or the ignition: only reading the fault codes settles it.

The most common causes

Diesel emissions systems: EGR valve, DPF, AdBlue

This is the leading cause on recent diesels. A clogged EGR valve whose flap no longer follows the command makes the airflow inconsistent and puts the engine into limp mode (Fiches-auto, Audi A6). A faulty particulate filter pressure sensor does the same, as on 2009-2010 Alfa Romeo MiTo diesels, which were the subject of a service campaign (CarVerif). On early Audi A4 B9 diesels, AdBlue tank or heater faults and NOx sensor faults lead to limp mode if not fixed (FixMyCar). See our guides to the EGR valve, a blocked DPF and AdBlue faults.

The turbo

A clogged variable-geometry turbo is enough to trigger limp mode (Fiches-auto, Audi A3). A dying turbo or a split boost hose cause a clear loss of power, often with a whistle: see whistling turbo.

Injection

Faulty injectors show up as hard starting, loss of power or rough running (CarVerif, Audi A4 2.0 TDI).

Ignition and timing (petrol)

Several failed coil packs can trigger limp mode on a petrol engine (GetCarwise, Aston Martin DB9). A faulty cam phaser or incorrect timing cause an engine light, loss of power and uneven combustion (Fiches-auto, Alfa Romeo Junior; CarVerif, Alfa Romeo Brera).

Overheating

The computer can also limit the engine when it runs too hot: an Alpine A110 owner describes frequent limp mode on track, where the car has to cool down (Fiches-auto). See temperature warning light and overheating.

What to do

Drive gently to a garage and have the fault codes read.

Before buying a used car

Loss of power or limp mode on the test drive is a reason to walk away, especially if the seller plays down an emissions warning or an AdBlue message (CarVerif, Audi A4 B9; CarVerif, Audi Q5 II). Test the car cold then warm, with at least one hard acceleration.