With a belt driven pump the flow weakens gradually, here the circulation stops in an instant. In urban traffic the temperature crosses the limit within minutes, the cylinder head distorts, and the repair is an engine rebuild.

Replacing a still good pump at around 120,000 to 150,000 km costs less than repairing an overheated engine. If the replacement is already in the service book, the pump has already been changed.

The short circuit and the fire can happen while the engine is switched off, and also while the car is parked. A car standing in a garage can catch fire too.

According to the German technical press, in January 2021 around 430,000 cars in Germany and more than half a million worldwide were recalled for this, all E90/E91/E92/E93 3 Series. In the USA more than 700,000 2006 to 2011 3 Series cars had already been recalled for the same thing in autumn 2017, after claimants in three separate cases cited injuries. The maker disputed that the fault had ever caused personal injury or an accident.

The EU Safety Gate names it in two separate alerts: the 3 Series (A12/00390/21) and the X5/X6 (A12/01261/21).

The campaign is free, but only if it was carried out. On a fifteen year old car, ask about it, because nobody will offer it unprompted.

Three things can happen. The oil stretches the belt and it can jump off (on a single belt engine that means an instant loss of the water pump and the alternator); oil getting into the alternator destroys the windings; and oil reaching the hot exhaust smokes and gives a burnt smell, and the official complaints do include burning smells and smoke.

The leak is visible: five minutes on a lift settles it. The gasket itself is a cheap part; the labour, the ruined belt and the alternator take the money.

The first is wear of the clutch pack and the fluid: in a slow, tight corner (a car park, a roundabout) the car jerks and "hops", as if the drive grabbed and let go. You can also get a jerk when pulling away on a slope. The second is the plastic actuator gear: the position signal is lost, the brake, ABS and 4x4/DSC lights come on together on the dash, and a clicking is audible from the transfer case when you switch the ignition off.

Workshops often propose a full transfer case replacement straight away. But the plastic gear can be replaced on its own, and the wrong tyre size or a mixed set of tyres also imitates the jerking, because the transfer case then slips continuously. So clear up the tyres and the fault code first, before you pay for a transfer case replacement.

A "compressor replacement" on its own often does not fix it: if the air spring keeps leaking, the new pump fails soon too. Air suspension is not fitted to every version, so the equipment list has to be checked separately.

The repair starts with finding the leak (soapy water, ride height sensor, valve block), not with replacing parts blind.

The symptom is irregular shifting, a gear slipping at higher revs, limp mode, a drivetrain fault code and a message on the display. The cause is the same in each case: the leak drops the hydraulic pressure, and the valve body cannot close the clutches with enough force.

The sleeve can be replaced without removing the gearbox, you only have to drop the oil pan. If it leaks for a long time, the slipping clutch and the overheating pump take the inside of the gearbox with them, and from then on it is a rebuild. The factory calls the oil a lifetime fill, but in practice the sleeve wears out around 60,000 to 70,000 miles, roughly 100,000 to 115,000 km.

The first is the mechatronics unit: the valve body and the control electronics in one. If the solenoids or the pressure regulation weaken, the gearbox jerks, the D/R engagement is delayed, it logs a gear ratio fault code, and finally drops into limp mode (often staying in 2nd or 3rd).

The second, and often misdiagnosed, is the connector sleeve of the mechatronics unit: the O-ring of the plastic sleeve ages, and ATF leaks from the side of the housing, exactly where the wiring harness enters. This is easy to mistake for a rear crankshaft seal. The trail starts from the driver's side of the housing, not from the clutch side.

On cars built in 2023 there was also a separate safety recall for a welding fault in the mechatronics unit. That is not the same as the wear related sleeve and the valves.

Check the fluid level, the area around the sleeve and the fault codes first; do not accept a "full gearbox replacement" quote straight away.

The sensor is usually the first to give up: oil leaks past its seal into the connector, and the oil corrupts the signal the engine control unit depends on. From then on the control unit gets no reliable position feedback, cannot set the valve lift precisely, and switches to a protective strategy. The fault code in this case is a plausibility fault, 2A67 or 2A6B: the control unit does not consider the position signal credible.

The eccentric shaft itself can also wear out or seize, and there are several causes for that: lack of lubrication from oil sludge and skipped oil changes, wear of the bearings, a system driven on with a failed actuator motor, and overheating and oil contamination in the cylinder head. It is reported from 150 000 kilometres.

The symptom is similar in all three cases: an unstable idle, stumbling and long cranking at a cold start, a check engine light that comes on and goes out on its own, lack of power or limp mode, inconsistent throttle response, and misfires cylinder by cylinder. A clicking, binding noise from the engine bay points to a seizing shaft.

The misdiagnosis is common, because the symptom shows up at combustion, so coils and spark plugs are suspected first, while the fault is in the air control. The condition of the shaft can only be established after the valve cover has been removed and it has been inspected. The sensor connector, though, is worth opening up first: if it is oily, that is the far cheaper route.

The fault is often not the "expensive motor" but the wiring fraying at the hinge. And that can only be found with the trim off. If the seller says "the spindle has already been replaced" but the wiring is damaged, the fault will come back.

This applies only to versions with the powered tailgate.