Please can anyone help with the following? I have a 2018 991 C4S which has a fault with the auto start/stop system. I bought the car new and the fault has been there since day one. Symptoms are that periodically it will fail to restart after stopping in traffic. Nothing works to get it restarted, including getting out of the car, locking and unlocking it and trying again. Weirdly, after 15 minutes of standing by the roadside it will then restart on the key. Sometimes the car will not start at all from warm. The fault appears completely randomly. In both cases it flashes up 'press brake pedal'...which clearly I am doing! It seems therefore to be brake related. The dealer cannot find the fault because nothing is appearing on the data. They have sent the data to Porsche Germany as well and they can't diagnose it. They have tried all sorts of things on the starting side of the engine. They have also replaced a pressure sensor on the brake master cylinder. Clearly if I could solve this by just turning off the auto start/stop I would but that doesn't help with the starting from scratch. Any ideas gratefully received (other than give the car back and get a refund, which I am tempted to do except for the fact that it is perfect for me and I can't get another one like it).
991 C4S starting/ auto start stop problem
- Thread starter JonathanC4S
- Start date
If it tells you to press the brake pedal when you already are doing so, it sounds to me like an intermittent failure within or poor location of the interlock switch on the brake pedal itself, assuming that the 991 is like previous models (need to check). I had exactly the same with the clutch switch on a new 987 - the starter was designed not to operate unless the clutch was depressed, and a faulty switch sometimes rendered the "depress clutch" message (even when it was depressed) and sometimes didn't. The OPC said that wasn't possible because there were no diagnostics indicating a dodgy switch, and if it was dodgy, it would always throw the "depress clutch" message (which it didn't). They eventually replaced the switch and bingo, all was well, until 6 months later it all happened again. "Impossible" said the OPC until I forced them to replace it again, this time with a new part number. After that, no problems. So if there is a switch on the brake pedal, maybe worth getting them to replace it, even if no diagnostics saying it has failed. Cheap and easy to suggest, at least.
Thank you very much Brian. That's great.
hi8ane
New member
My 2014 C4S had a similar problem which culminated in it refusing to start on my drive. The AA chap disconnected the battery and gave it a full boost from his van which did result in the car restarting but his advice was to get it to an OPC asap. A Senior Technician at my OPC found that the factory-installed Tracker had a flat battery, confirmed when he contacted Vodafone: they could "see" that there was no longer any power to the unit so it put the car into full disabled mode. A new unit under warranty solved the problem.
ETA - I didn't subscribe to the Tracker service as my insurer didn't require it. As a result the unit didn't install over-the-air updates fixing a known internal battery charging issue. The new unit doesn't have this issue apparently.
ETA - I didn't subscribe to the Tracker service as my insurer didn't require it. As a result the unit didn't install over-the-air updates fixing a known internal battery charging issue. The new unit doesn't have this issue apparently.
This happened to me when I was using my wife's the BMW320d a few years back.
At the front of the traffic light queue and it refused to start. After being stuck there in rush hour traffic for over an hour, causing traffic to back up the M6 and back 2 miles into Preston I was recovered by BMW Assistance.
The fault turned out to be the 'brain' and therefore a new computer was fitted under warranty. Since then I have always hated the stop start mechanism in all my subsequent cars..
At the front of the traffic light queue and it refused to start. After being stuck there in rush hour traffic for over an hour, causing traffic to back up the M6 and back 2 miles into Preston I was recovered by BMW Assistance.
The fault turned out to be the 'brain' and therefore a new computer was fitted under warranty. Since then I have always hated the stop start mechanism in all my subsequent cars..
Thanks for the replies on this one. Really helpful. One other thing worth adding is that the fault is intermittent and when the car won't start the brake pedal is noticeably harder than usual. After 15 mins or so when it will start again the brake pedal is softer. I was wondering if it might be ABS related, although there are no warnings to indicate that it might be.
hi8ane
New member
Mine was intermittent too so if I remembered I generally disabled Stop/Start from first start. Can't say I noticed brake pedal pressure when it failed as I was more concerned with getting it restarted! May or may not be relevant but my Volvo V90 and XC90 had hard brake pedals when the engine was off due to the "Apply Parking Brake when Engine Off" function. Don't know if the 991.2 has this (the 991.1 doesn't).
Just to add, when mine failed to restart, the turn of the ignition key had zero effect - no click, nothing - which helped diagnose the immobiliser fault.
Just to add, when mine failed to restart, the turn of the ignition key had zero effect - no click, nothing - which helped diagnose the immobiliser fault.
Posts made and opinions expressed are those of the individual forum members
Use of the Forum is subject to the Terms and Conditions
