Ford Focus Undervoltage Issue

Sounds familiar. I had similar issues with my Focus. Voltage drop can cause all sorts of weird electrical problems. Have you checked the battery's voltage with a multimeter while the engine is running? It should be around 13.5-14.5 volts. Also, how old is the battery itself?

Thanks for the quick reply. I haven't checked the voltage while the engine is running, only when it's off, and it reads around 11.8V. The battery is about 4 years old, so getting on in years. What exactly was the problem with your Focus when you had these symptoms?

With your voltage reading, I'd say it points to a defective power supply. Same thing happened to me, the system was showing low voltage. After some investigation I brought mine to a workshop. They load-tested it and the battery failed. Replacing the battery solved it. Given your battery's age and those symptoms, that would be the next logical step.

Thanks for confirming. I suspected the battery, but wanted to rule out other possibilities. I took your advice and brought it to a workshop. They confirmed the battery was heavily discharged and defective. Replacing the battery (cost 290€) resolved the undervoltage issue and all related symptoms.