Check the fuses in the engine electrical system.
Is the main fuse (slot 1) for the on-board supply OK?
If there are no defects, then check the voltage on the glow plugs with a multimeter.
If there is at least 11.5 V, do not miss the opportunity to check the glow plugs.
To do this, disconnect the wire on one of the four glow plugs.
Connect a test lamp between the ground and the disconnected wire.
Disconnect the coolant temperature sensor connector and, having protected it, put it away.
Turn the ignition key to the "Vorgluhen" position - pre-heating.
If the control lamp now lights up for about 20 s, the voltage at the glow plugs and the pre-glow time set by the control unit are OK.
If the test lamp does not come on, check the main fuse or pre-glow relay.
If the pre-heating time is not reached, then look for a defect in the control unit.
Faulty pre-heating system
You will probably only notice defects in the pre-heating system in very cold weather (below -10°C). Only then will the diesel engine be reluctant to do its job without external heating energy. With careful inspection, you can recognize possible defects in advance: pay attention to the exhaust gases of your Focus. If blue plumes of smoke are coming out of the muffler, then for prevention, read the fault memory device in your workshop - blue smoke is a signal of cold combustion. Small clouds should not bother you.