What Fiat codes (P0420) actually mean on Fiat engines — the real cause, the common misdiagnosis, and the honest UK repair cost.
These are the OBD fault codes we see most often on Fiat vehicles in the workshop, with the real cause on Fiat engines rather than the generic textbook definition, plus the honest independent-garage cost.
Fiat fault codes explained
- P0420 — Catalyst efficiency below threshold on Fiat — 1.2/1.4 8v and TwinAir cat aging plus oil-burn from worn valve seals. Typical UK cost £180–£650.
Codes are generic across manufacturers, but the fix is not. The same code on a Fiat usually points at a different component than it would on another make, which is why a make-specific diagnosis saves money before any parts are bought.