The sluggish performance and the erratic tachometer needle may not be related at all. That said, they could very well both be symptoms of the same problem. A lot of mechanics, when shown an erratic dashboard gauge, will start by checking for a bad earth point. A poor earth can lead to all sorts of problems, as a healthy flow of electricity is vital to the basic functions of any modern car.

But before you rush out to find an auto electrician, bear in mind that this sounds very much like a problem that would be covered by Mazda’s factory warranty. Provided you have had the car serviced correctly and it hasn’t been submerged in a flood or something equally drastic, the first step is to take it back to Mazda and allow the dealer to tackle the problem.

In the old days, this would (probably) have been a simple matter of a faulty wiper switch. But on modern cars, functions like the wipers are controlled by a computer called the body-control module. If this starts to fail it can send odd signals to the various things it controls (wipers, lights, central locking, climate-control, entertainment systems and many more).

An automotive electrician might be able to pin-point the problem, and that’s a good place to start. But before you do that, try this: Disconnect the car’s battery and leave it overnight. When you reconnect the battery, you might just find the computer has reset itself and things return to normal. It can sometimes be just a temporary fix, but it’s definitely worth a try. And it’s free.

Mazda (and other car-makers) have been having lots of problems with the oil level rising in the engines of some of their diesel models. The problem is caused by the Diesel Particulate Filter attempting to regenerate itself. In an ideal world, these diesel vehicles would be driven for at least an bour or so at freeway speeds at least once a month. That would get the exhaust system hot enough for the DPF to regenerate on its own (burn the contaminants from the exhaust to ash and expel them, basically).

But in cars that don’t see this sort of use and are more commonly driven around the suburbs at moderate speeds, the exhaust never gets hot enough for this to occur. So, the solution has been to inject extra diesel into the engine to make the exhaust hotter and, therefore, get it all to a temperature where DPF regeneration can occur. The problem is that some of that diesel can get into the sump of the engine where it dilutes the engine oil. And that’s why you’re seeing the level rising on the dipstick over a period of weeks and months.

Diluted engine oil is bad news because it may not lubricate and protect the engine as well as non-polluted oil. The warning light you’re seeing is telling you that you might be at that point and that an engine oil change is probably a good idea, even if the service interval hasn’t been reached yet. Just draining some of the contents of the sump away is not the answer as the oil is already diluted by the diesel.