Let’s start at the start. The SVS light on your car stands for 'Service Vehicle Soon'. Fundamentally, it’s telling you that something is not right somewhere on the vehicle and the computer has logged the fault. That’s why a scan showed up the code PO661.

On this vehicle, code PO661 relates to the electronics that control the swirl valve inside the intake manifold. This valve works to create the maximum gas velocity inside the intake runners and, therefore, gain the cleanest, most complete burn of the fuel and air entering the engine. If this valve is not operating properly, you may find a loss of power as the turbocharger won’t be working at its most efficient. In some cases, the same fault code can show up because of a problem with the engine’s EGR valve which is also part of the engine’s intake system. This may require the manifold to be removed and manually cleaned.

However, it’s unlikely to be anything to do with the car’s air-conditioning system, which is more likely to be a coincidence. That said, if the engine is running hotter than normal because the intake system is not working properly, then the car may have an interlock that prevents the air-con coming on to help keep coolant temperatures at bay.

The first thought here is that you’re dealing with a dodgy throttle-position sensor or some other sensor that is giving the on-board computer a reason not to exceed 2500rpm. Or tricking the computer into thinking that the engine is spinning faster than it really is. Have you had the vehicle scanned? It’s a cheap way of reducing a lot of the guesswork in a case like this.

Like any engine, of course, a turbo-diesel won’t rev beyond the speed that the fuel supply can support. You say you’ve changed the fuel filter, but have you checked the pump pressure and the fuel-delivery rate? A problem with the pump or fuel lines could easily produce the symptoms you have. You could even have a blocked fuel pick-up in the tank.

The other problem with modern turbo-diesels is that they are prone to clogging their intake systems with a black gunk that is a by-product of soot and oil mist from the vehicle’s exhaust-gas recirculation and crankcase-ventilation systems respectively. This black, ooze can sometimes almost completely block the intake path for air entering the engine and will cause all sorts of dramas, including the one you’re seeing.

The D-Max is pretty well regarded in the trade for its ability to go the distance, but modern, common-rail diesel technology has shown that a vehicle with fewer kilometres is usually a better bet than one with more. Although they do an amazing job in terms of power, torque, towing and fuel economy, today’s turbo-diesels are pretty highly strung in some ways and really need their maintenance. And the older they get, the more attention they seem to need in terms of new injectors, filters and pumps.

A D-Max with those kilometres might be ready for a pretty big (and expensive) service, too, so make sure your first trip in it isn’t going to be to a workshop. Ultimately, price, condition and service history should steer your decision as they should in any second-hand vehicle purchase. I’d take a vehicle with 150,000km with a full service history over a 60,000km one with no service records.