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.

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.

What the owner notices is that the coolant goes down, but there is no puddle. If the seal has let go, the loss becomes rapid, and the low coolant warning appears as well. The fault appears above 120 000 kilometres and can lead to overheating.

The factory repair is outstandingly expensive on this engine: to get the pipe out, the timing chain cover, the chain, the intake manifold and the cylinder head all have to be disturbed, and that is over 40 hours of work, with a bill above 4000 dollars. That is exactly why two aftermarket routes exist: a liner pushed into the old pipe, at around 1000 dollars, and a collapsible pipe that can be fitted without opening the engine, at between 1500 and 2500 dollars.

One thing is worth asking. Workshops usually replace the gasket of the alternator bracket and the seals of the valley pan in the same job, because they are reached from the same place. If the pipe has been done, the other two are worth looking for on the invoice.

This fault is dangerous because it gives no advance sign. There is no warning signal from a disintegrating impeller, and the engine temperature runs up faster than the gauge can follow; by the time it shows, the cylinder head can already be damaged. The breaking impeller is a fault of its own on the six-cylinder built between 1992 and 1995, which is why the maker went back to a metal impeller, and then used a composite impeller in the later engines, which proved more durable. A pump with a metal impeller is still available today, as a durability replacement.

There is, however, one sign that can be caught before the failure: play developing in the shaft of the pump. That means the bearing is wearing, and because the impeller sits on this shaft, play in the shaft signals the coming end of the pump. With the belt off it can be felt by hand.

On the M54 it appears at around 120 000 and on the N62 at around 150 000 kilometres. Once the engine has overheated, the consequence stays: a warped cylinder head and a damaged gasket are no longer put right by replacing the pump.

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 characteristic sign is a puff of blue smoke, and not only at a cold start: it also comes when the car has idled for a long time at a light, or when it is started after standing for a while. Oil consumption rises slowly but steadily alongside it. The valve guide often wears together with the seal, so the repair does not necessarily end with replacing the seals. The fault appears above 130 000 kilometres.

The cylinder head does not have to come off for the repair. If the workshop has a valve spring compressor that can be used with the head in place, the camshaft and the chain do not have to be disturbed either, but the job is still a long one. The shop price is between 1800 and 3000 dollars, while the parts themselves are 300 to 500 dollars by comparison.

The symptom is a visible trace of oil around the alternator, and this is the rarer case: here the leak also appears from outside, instead of only collecting in the V valley. Fresh, shiny oil around the alternator belongs here.

The gasket itself is under 10 dollars, but the labour is not. For the repair the alternator has to come off, the power steering pump has to be unbolted, the engine has to be lifted and the driver's side engine mount has to be removed as well. That is between 7 and 10 hours of work, with a bill of between 1000 and 1500 dollars, and workshops also allow for removing the intake manifold.

Because they are reached from the same place, the coolant pipe running in the V valley and the seals of the valley pan are usually done in the same job. If only one of them is on the invoice, it is worth asking why not the other two.

The early sign is a rattle audible at a cold start from the front of the engine. If the chain jumps, the valves and the pistons meet, and that is already an engine rebuild.

The condition of the guide can only be seen with the engine apart, so the rule is that whenever the upper timing chain cover is off, the guides have to be looked at as well. That comes up during the repair of the coolant pipe running in the V valley and of the alternator bracket, because those require opening up too.

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.