I have a PO421 code (warm up catalyst efficiency below threshold bank 1). Does anyone know what part/s would need to be replaced to fix it? I think it could be the catalytic converter, but it may be something simpler (O2 sensor?).
Any help would be appreciated. Thanks, Rich

All Diesel Fleet:
1985 Mitsubishi 2.3 Turbodiesel Pick-up, 1983 GMC Van- 6.2 Diesel.
Benzs: 1995 E300 diesel, 1980 300SD, 1982 300CD, 1983 300D, 1985 190D.
1980 AMC Eagle Limited Wagon powered by: 3.6 Litre I6 VM Turbo Diesel Engine (one of fewer than 10 made, only 2 known to still exist)