What the code means — and where to see the likely cause on your engine.
OBD-II diagnostic trouble codes are standardized by SAE J2012, so a P0420 means the same thing on any car — the catalyst is reading below threshold. What differs is the likely cause and fix, which depend on your engine and emissions system. The reference below is the universal definition; open your vehicle's page for the codes and diagnostics specific to it.
AcuraAlfa RomeoAmerican Motors
Audi
BMW
Buick
Cadillac
Chevrolet
Chrysler
Dodge
Eagle
Fiat
FordGMC TruckGenesis
Geo
HUMMER
Honda
Hyundai
Infiniti
IsuzuJaguar
Jeep
KiaLand Rover
Lexus
Lincoln
MazdaMercedes
MercuryMerkurMini
Mitsubishi
Nissan
Oldsmobile
PlymouthPolestar
PontiacPorscheRam
Saab
Saturn
ScionSmartSubaruSuzukiTesla
Toyota
VolkswagenVolvoOBD-II code — a five-character diagnostic trouble code: a system letter (P powertrain, B body, C chassis, U network) followed by four digits.
P0 vs P1 — a P0 code is standardized by SAE and identical on every make; a P1 code is manufacturer-specific.
Freeze frame — the sensor snapshot the computer stores at the moment a code sets, used to reproduce the fault.
Pending vs stored — a pending code needs one more failed drive cycle to confirm; a stored code has already turned the check-engine light on.