Before you buy a used BMW X6 (E71/E72).

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: 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: 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: The 4x4, ABS and brake warning lights come on together while driving (often intermittently), a click or clunk under the car when switching the ignition off or on, and jerky acceleration in low gears.

Ask them to check: Scan every module with a diagnostic tool (transfer case codes) and measure the tread depth on all four tyres. The ABS and brake lights can have other causes, such as a wheel sensor or the ABS module: confirm before replacing parts. With the brake light on, also check the fluid and do not drive if the pedal feels abnormal.

Look out for: High oil consumption, blue or white smoke on start-up, misfires, chain rattle and a battery that drains.

Ask them to check: Oil consumption test, confirm by VIN that the Customer Care Package was done and listen for the chain on a cold start.

Look out for: Chain rattle for the first 5–10 seconds after starting; on engines with low oil pressure, noises and oil pump damage.

Ask them to check: Listen to a cold start and check the oil change history. The gearbox must come off to replace it.

Look out for: Harsh shifts (especially engaging D and 1st to 2nd), slipping or jerking; oil leaks from the pan, the mechatronic connector sleeve or the seals.

Ask them to check: Check the oil level with the official procedure (correct temperature and engine running), look for leaks at the pan, connector sleeve and seals, inspect the pan for impact damage and review adaptation values with ISTA.

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.