The 2021 Mazda3 uses Mazda's Skyactiv architecture and a diesel will have a DPF and EGR system — it rewards steady motorway use rather than lots of short urban trips. As a manual-diesel, clutch wear and turbo health are the usual items to watch on a 5-year-old example; diesel Mazdas are otherwise straightforward to service and parts are readily available through dealers and aftermarket suppliers.

• DPF behaviour: signs of frequent forced regens, persistent DPF warning light or heavy soot on the tailpipe (common if used mainly for short trips).
• EGR/intake soot: rough idle or loss of low‑end power can point to a clogged EGR or intake ports.
• Turbo health: listen for whining, check for blue/black smoke on acceleration and inspect for oil leaks or shaft play.
• Clutch condition: check biting point, any slipping under load or a high clutch pedal (manual wear expected at 5 years).
• Gearbox syncros: smooth selection of 2nd/3rd and no crunching when changing gears, especially if urban stop‑start use is common.