The water-cooled flat-six behind the original Boxster and 996-generation 911 — genuinely excellent to drive, but carrying the single most consequential and widely discussed reliability issue in modern Porsche history.
The intermediate shaft (IMS) bearing supports a shaft that helps drive the camshafts, and on a meaningful share of these engines, that bearing can fail — sometimes with little warning, and when it does, the resulting damage is typically catastrophic and requires a full engine rebuild or replacement. Porsche revised the bearing design over the engine's production run, and later M97 examples carry somewhat lower risk than early M96 units, but no version of this engine is entirely free of the concern. This is, without real exaggeration, the single most important thing to understand before buying any car with this engine.
Not a guarantee, but a pattern seen consistently across owner forums and independent-garage reporting.
IMS bearing failure has been documented across a very wide mileage range, including relatively low-mileage examples — it is not purely a high-mileage concern.
A documented, upgraded example is a meaningfully different, much safer proposition at any mileage.
This is absolutely non-negotiable: ask whether the IMS bearing has been proactively replaced with a revised aftermarket solution, and get documentation if so.
If unaddressed, budget for the preventative upgrade as an essential near-term cost, not an optional extra — this is standard practice among informed Porsche buyers.
A pre-purchase inspection by a Porsche specialist familiar specifically with this issue is worth the cost given how much is at stake.
Model-specific pages, with notes on how it behaves in that particular car.