BMW E90 diesel won't start in cold weather
Why does an E90 diesel not start in frost?
The most common cause is the glow system: a glow plug has burnt out or the glow plug control unit (glow box) is broken. Fault codes point to a glow fault. The glow plugs should be measured, because some may be dead before the symptoms get worse. A sensor showing the wrong temperature can also prevent glowing.
What else should be checked if the glow plugs are fine?
Check the battery, starter motor and wiring. A weak start cranks the engine too slowly. The positive lead can be oxidised through at its terminal, and there can be frozen water in the battery tray. The earth lead between the engine and the body, and the control unit's earth point, can also oxidise and cause interruptions.
What if the cause is the injectors?
If the glow plugs, battery and wiring are fine, the injectors can be the cause. The glow relay's internal wires can also burn through, and the relay is repaired by soldering or replaced with a new one. An injector fault requires investigation by a workshop.