Question of the week: How can I fix my faulty stop start system?
Fri, 27 Mar 2026
Dear Honest John,
"I have a 2021 Mazda6 2.0 petrol and the stop/start function is not working. This first happened on 10th January 2025. I became aware that the stop/start function was not functioning; no warning lights, just not working.
I had been on a 3 week holiday from the mid November 2024 to early December 2024 and left the car on a smart charger. When I returned the car started first time and the stop/start function was working. However, in early January 2025 it stopped working. As the car was covered by a Mazda UK warranty I took it to my local Mazda dealer who kept it for 24 hours. Diagnostic tests showed zero faults and they charged the battery overnight. They returned the car to me, but the stop/start function was still not working and the dealer was not sure as to why...
As the battery was original I decided to change the battery - despite the dealer saying the original battery was indicating 100% health - and the dealer fitted a N65 Varta Blue Dynamic EFB Start-Stop Car Battery 12V 65Ah (565501065) Type 005L battery supplied by me. That cured the issue.
However, 12 months later the problem has re-occurred. I went on holiday from 16th November to 17th December 2025 leaving the car hooked up to the same smart charger, and when I returned the car started and worked normally until early January 2026 when the stop/start function ceased working. I can switch off the stop/start function and the orange warning light illuminates. I then switch on the stop/start function and the orange warning light goes out, but the stop/start function does not work.
I have had both the old and new batteries tested (I kept the old battery on charge with the smart charger) and they both show as being 100% and there are no fault codes showing when plugged into the diagnostic equipment; this was carried out by my local independent garage as Mazda wanted to charge me £90.00 to plug my car into their diagnostic tool - the cost of a new battery...
I have asked both my local Mazda dealer and the Mazda dealer (Stourbridge) where I purchased the car about the issue and both said that they don't know why the stop/start function is not working as it should. I have booked it in with the Stourbridge Mazda dealer for a service in April and they said that they will investigate the issue at the same time, but hinted that only a Mazda original battery (there is no such thing as Mazda don't manufacture batteries, but buy them from Yuasa) will resolve the issue. To supply and fit a new "Mazda" Yuasa battery will cost £265, which I'm reluctant to agree to.
The car is used on long runs and only the odd short run, so the battery gets lots of time to be charged by the car. I have plugged it into a smart charger, since January 2026, to see if that will resolve the issue, but it has not. Do you have any suggestions?
- IH
Dear IH,
The fact that there are no fault codes and several of the batteries that have been fitted to the vehicle have been tested and shown as healthy, yet the stop start system continues to be intermittent would suggest that the issues lies with the car's system monitoring the battery's state of health.
The stop start system is designed to operate whenever possible, but will not do so if it believes that there is insufficient charge in the battery. One possibility is that the battery was not coded to the vehicle when installed which is where the car is 'told' that a new battery has been fitted, however we would expect this to be part of the battery replacement routine for a Mazda dealer.
There is a manual reset procedure that you can carry out which as follows: 1) Ensure the ignition is switched off 2) Switch the ignition to the on position but do not start the car 3) Within five seconds of switching on the ignition, hold down the stop start button for three seconds 4) Start the car, and press the stop start button which should make the warning light flash 5) Allow the engine to idle until the warning light goes out 6) Switch the engine off and wait three minutes 7) Start the car and drive in the normal way - if functioning correctly the stop start system should engage after a few miles.
If this has been carried out, we would suggest there could be a fault with the sensors or ECU that monitor the battery health. A reset or software update for these systems may be sufficient, but we would suggest taking your vehicle to either an independent Mazda specialist or automotive electrician to have this system tested.
It may also be worth investigating if there is a battery drain, where a vehicle component is not shutting down correctly when the vehicle is off causing the battery charge to drop below the levels where the stop start can activate.
