Mercedes E Klasse Hybrid Rear Window Heating Issue

I had a similar issue with my old 2018 BMW 5 series diesel where the rear window demister stopped working. I thought it was a fuse, but it was a faulty heating coil. Have you checked the voltage at the rear window heating element connector when the defroster is on? Is the error code specific? Knowing the code helps. What's the battery voltage when the engine is off and when it's running? A weak battery can cause electrical issues, especially in a hybrid.

Thanks, I will check the voltage at the rear window heating coil connector. The error code indicated a general fault in the rear window heating circuit. The battery voltage seemed okay, but I'll check again. What was the error code with your BMW?

My BMW's error code was an open circuit in the rear window heater, not battery voltage. Your generic code and issues with both heaters suggest a battery problem. Hybrids might be sensitive to voltage changes. If the voltage is okay, the battery's internal resistance under load might be the issue. Get a battery load test at a workshop. They can apply a load and see if the voltage drops too much. I'd recommend a professional diagnosis based on your symptoms.

Thanks for the advice. A Mercedes-Benz workshop confirmed a discharged auxiliary battery was affecting the rear window heating. Replacing the battery fixed it. The cost was 290€. Your help was valuable.