For a start, this model Mazda BT-50 did have a recall to check the transmission’s output shaft speed sensor. If this failed, the transmission could suddenly shift back to first gear which, if it did so at highway speeds, would have been potentially disastrous. But it’s also possible that this sensor could fail and not allow the transmission to shift beyond first gear, so that would be worth a check. A transmission specialist is your best bet here.

Beyond that, simply servicing a component won’t generally fix a fundamental problem. It would be the same with a worn out or internally damaged engine; servicing that engine with fresh oil and spark plugs will not fix the underlying condition.

The problem of a clutch pedal hitting the floor without actually disengaging the clutch is usually caused by either a faulty clutch master cylinder, slave cylinder or a loss of clutch fluid from the system.

These are not expensive or complex things to fix and new master cylinders and slave cylinder are readily available from parts stores and online sellers. However, if the problem is a lack of hydraulic fluid, you also need to figure out where the old fluid went. There must be a leak somewhere and until that’s fixed, topping up the fluid will only be a temporary fix.

If you’re really unlucky, you might find the clutch has collapsed internally, at which point a whole new clutch will be needed. This vehicle also used a dual-mass flywheel, so you’ll often need to replace that as well to do the job properly.

Sometimes head units need a reboot or a return-to-factory-settings button to be pressed. Often, there will be a tiny hole somewhere on the face of the head unit. Into this, you insert a thin piece of wire or a toothpick (or similar) and press just hard enough for the screen to go blank and the unit to reboot. It sounds like you’re using the factory Mazda reversing camera with an aftermarket head unit, so you may need to visit a car audio specialist to get it all working together.

Changing a car’s battery can often be enough to scramble the settings on things like cameras and stereos. You may need to find the security code for the head unit to make it all work properly, too.