Mazda BT-50 Problems

You shouldn’t be using Park as a brake, that’s not what it is intended for, and could damage the transmission. If you want to stop the vehicle change down through the ratios manually and apply the brakes. The problem is unlikely to be the wiring harness, it sounds like it’s deeper within the transmission. Have your dealer check it, or take it to an automatic transmission specialist.

You need to take it to a mechanic for a diagnostic check to find the cause of the problem. It sounds like it could be in the transmission; perhaps the powertrain module could be at fault.

Without being able to view it and hear it firsthand it's hard to suggest what might be wrong. It could be a bearing in the turbo, but that's only an educated guess. I would have a mechanic check it.

It’s unclear from the information you have given us just what the problem is, but I can make a guess that the belt is not tightening around you when you put it on. Whatever the problem take it to your Mazda dealer and have it checked, it could be a fault with the retractor; it could be a problem with the webbing.

You should be running a 5W-40 oil.

It is a relatively common problem, and it’s made worse by towing in 5th gear.

On average a 2010 model should have done 200-250,000 km, so you are correct in thinking it is high. The BT-50 is generally reliable, but having done almost 400,000 km you have to hope for the best and be prepared for the worst. Having a good service record is a bonus.

As there are no warning lights, and it recovers if left, it doesn’t seem likely that it’s an electronic issue, rather a mechanical one. It would worth checking the turbocharger.

I’d be checking the clutch to see if it could be worn out or close to.