My 2012 Mercedes-Benz E-Class has only done 21,000 miles. I've had it from new with the original battery. It stood idle for three weeks and was sluggish to start yesterday. I switched on the heated rear windscreen and blower to clear frost but the gear selector would not move from P. I ran the engine for a few minutes but the fault persisted. Later that day I started it again, this time I was able to select gears as normal. I ran the engine for 20 minutes last night and the gear selection is okay today. Could low voltage and a failing battery cause this?