Mercedes-Benz W203 ignition switch does not recognise the key
Why does the W203 ignition switch not recognise the key in the cold?
One common cause is the fibre optic inside the ignition switch, which collects dust and moisture. The dirt blocks the infrared signal, and the fault gets worse in frost. Poor contact in the ignition switch connectors can also cause the symptom. The fault can be in the switch even if no fault codes are found.
Can the fault be in the key alone?
Yes. The coil at the tip of the key can come loose or its solder joints can break, so the key gets no power from the lock. The key's circuit board can also be broken. Try the spare key: if it works, the fault is in the key and not in the lock.
What is checked when the ignition switch does not turn and the car does not start?
Check the ignition switch fuse and the fuse connections, because oxidised or poorly contacting points can cut the supply. The ignition switch identification can be read with a diagnostic tool even when the ignition cannot be switched on. If the lock does not work, a workshop can replace or service it.