A dashboard warning light referencing the DPF (diesel particulate filter) or EGR (exhaust gas recirculation) system is, by a wide margin, the single most common dashboard warning across every diesel engine covered on this site — and in the large majority of cases, it traces back to one specific factor: how the car has actually been driven.
A diesel particulate filter captures soot from the exhaust and needs to periodically burn it off at high temperature — a process called regeneration — to keep working. That temperature is really only reached during sustained, higher-speed driving. A car used mostly for short trips around town never gets hot enough for long enough to complete that self-cleaning cycle, and the filter gradually clogs. The EGR valve, which recirculates some exhaust gas to reduce emissions, suffers a related but separate problem: it's exposed to that same soot and carbon buildup, and can gum up or stick over time, especially on cars that see mostly low-speed, low-load driving.
This is why the exact same engine can be a genuinely dependable choice in one buyer's hands and a source of recurring warning lights in another's — the underlying hardware is often identical; the driving pattern is what differs.
This symptom shows up across several unrelated engines and drivetrains — each has its own root cause and its own full writeup.
Ask directly about driving pattern before buying any used diesel — a car that's mostly done short urban trips carries meaningfully more risk than one driven mostly on the motorway, regardless of mileage.
If the light is already on, a sustained highway drive (20-30 minutes at consistent speed) can sometimes trigger a successful regeneration and clear it on its own — worth trying before assuming a repair is needed.
A pre-purchase inspection that reads live data during a drive will catch early-stage DPF loading before it becomes a dashboard warning at all.