How to choose a reliable used diesel car
Choosing a reliable used diesel isn’t simply about finding a car with the lowest mileage. Modern diesels combine the engine with emissions equipment, turbochargers, increasingly sophisticated gearboxes and, in some cases, complicated drivetrains.
A sensible approach is to choose a proven, relatively simple diesel powertrain that suits your driving, then concentrate on the condition and history of the individual car.
Does a diesel suit your driving?
Start with how you’ll actually use the car.
Diesels remain well suited to regular longer journeys, particularly where their good motorway economy and low-speed torque are useful. Repeated short journeys will ultimately be harmful to the emissions systems such as the DPF and EGR. Over the long run, this will increase your chances of being hit with a hefty repair bill.
Choose the engine, not just the car
Don’t assume that every diesel version of a particular model is mechanically similar.
Manufacturers often offer several power outputs and sometimes substantially different engines within the same model range. If reliability and long-term costs matter more than outright performance, a proven, modestly powered version is often worth considering.
In general, look for an engine with moderate power output. The higher the power output, the more complex the engine hardware is to obtain that output. With complexity comes higher repair costs.
DPF, EGR and AdBlue
Modern diesel reliability isn’t only about the engine.
A typical diesel will contain a DPF and exhaust gas recirculation (EGR), while many later cars also use AdBlue/SCR equipment and NOx sensors. These systems reduce harmful emissions but introduce additional components that can develop faults as a car gets older.
Most modern used diesels will use a DPF and EGR, but many will not use Adblue/SCR. If you can, aim to avoid cars using Adblue systems. Want to know why? See our article.
Think about the gearbox and drivetrain
The gearbox can be as important as the engine.
Automatic gearboxes are much more mechanicaly complex than manual transmissions. Many automatic transmissions were poor straight out of the box and are potentially expensive ticking timebombs.
In general avoid cars with dual clutch transmission (DCT) as these require relatively frequent, expensive oil changes and are inherently less reliable that torque converter gearboxes, which are the gold standard for automatic gearboxes.
The ZF eight-speed torque-converter automatic is widely used and has developed an excellent reputation. It was widely used several manufacturers including by BMW, Jaguar, Land Rover and in some VAG group models.
You may see a cars with all wheel drive systems (branded X-drive by BMW or Quattro by Audi for example). Generally avoid these systems as they add an extra differential requiring maintenance and these systems can be damaged by mismatched tyres.
Don’t pay for complexity you don’t need
Overall the key is the reduce your potential exposure to mechanical repairs by avoiding unnecessary complexity.
Multiple turbochargers, four-wheel drive, air suspension, adaptive dampers and other sophisticated equipment can all provide genuine benefits. They also give an older car more components that can potentially require repair.
There’s nothing wrong with choosing these features if you want them. But if your priority is dependable, economical transport, the simpler version of the same car would make more sense.
History matters more than low mileage
Maintenance and servicing apply to all used cars, including diesels.
Don’t automatically choose the lowest-mileage diesel you can afford; a poorly maintained low mileage car may be in a worse mechanical condition than a higher mileage well maintained car.
Look for evidence of regular oil and filter changes, appropriate gearbox maintenance and significant previous repairs. Check the MOT history and whether relevant manufacturer recalls have been completed.
When viewing the car, ideally start it from cold. Watch for warning lights, unusual smoke, rough running or abnormal mechanical noises. Make sure the gearbox behaves properly and consider a diagnostic scan or independent inspection for a more expensive purchase.
Miles Better Off used-diesel checklist
Before buying, check:
- Does your normal driving actually suit a diesel?
- Which exact engine is fitted, and does it have significant known weaknesses?
- Which DPF, EGR and AdBlue systems are fitted?
- Which gearbox does it use, and does it have a good maintenance history?
- Are you paying for mechanical complexity you don’t actually need?
- Is there convincing evidence of regular servicing?
- Have important scheduled maintenance items been completed?
- Are recalls outstanding?
- Does the MOT and repair history suggest the car has been looked after?
- Does the individual car start, drive and behave as it should?
The Bottom Line:
There isn’t a manufacturer or engine that guarantees a reliable used diesel. Choosing a proven powertrain that suits your driving, avoiding unnecessary complexity and then finding a well-maintained example gives you a much better starting point than simply shopping by badge, power output or mileage.