Before you buy a used BMW Serie 3 (E90/E91/E92/E93).

Look out for: Chain rattle on cold starts, VANOS engine light, rising oil consumption and, in severe cases, the engine seizing with no prior warning.

Ask them to check: Listen to a cold start, check the oil filler cap for plastic debris and read VANOS codes.

Look out for: After a flat battery, a battery change or disconnection, or a jump start: lights, indicators or windows stop working, side or dipped lights stay on and diagnostics cannot communicate with the module (for example code 00A3C1).

Ask them to check: Check the battery and charging and try communicating with ISTA. If the module does not respond, it can be repaired by rewriting its EEPROM (specialist services or programmers) instead of being replaced, then coded. Use a voltage maintainer when working on the car.

Look out for: Temperature warning and limited power with no prior warning (water pump); burning oil smell or smoke from leaks onto the exhaust or belt; rough idle and VANOS codes (2A82, 2A87).

Ask them to check: Check the water pump and thermostat history, look for leaks at the valve cover and oil filter housing, and read VANOS codes.

Look out for: Loss of power, smoke or knocking, often with no prior warning: one of the flaps comes loose and drops into the cylinder.

Ask them to check: Check whether they have already been removed or replaced with blanking plugs and inspect the intake manifold.

Look out for: Usually none until it fails. A common early warning: oil seeping around the flap spindles on top of the engine. If a flap or its screw comes loose and enters a cylinder: sudden heavy knocking, loss of power, smoke and possible turbo damage as well.

Ask them to check: At a garage: confirm whether the manifold has swirl flaps (some versions do not), look for oil around the spindles and play in the flaps, and check the history for earlier replacement or removal. With any new metallic knocking, stop the engine and do not restart it.

Look out for: Rattling from the rear of the engine (gearbox side) on a cold start, getting worse over time. In extreme cases the engine cuts out while driving.

Ask them to check: Listen with the engine fully cold, ideally with a mechanic's stethoscope on the rear of the block.

Look out for: Long cranking, hesitation, engine light and limp mode (high-pressure pump); metallic rattle at idle or between 1,500 and 3,000 rpm and low boost (wastegates); misfires from injectors.

Ask them to check: Read fuel pressure and boost codes, listen at idle and confirm by VIN that the revised pump is fitted.

Look out for: Little prior warning; once wear is advanced, a metallic rod knock that rises with revs. On the S65 and S85, also jerking and limp mode from failing throttle actuators.

Ask them to check: Oil analysis and bearing replacement history. Indicative preventive replacement: S54 around 130,000–160,000 km, S65 around 100,000–130,000 km and S85 around 80,000–110,000 km.

Look out for: Oil drips or dampness by the electrical connector on the side of the pan, a bump on the downshift just before stopping, harsh or flared shifts (especially 3rd-4th and 4th-5th under throttle) and a limp mode that clears after restarting.

Ask them to check: At a garage: read the gearbox codes, check the fluid level with the official procedure (correct temperature and engine running) and look for a real leak at the sleeve and pan gasket. After a fluid or parts change, the garage should reset adaptations. If limp mode persists, check solenoids and valve body before replacing the gearbox.