Diagnostic trouble code P0421 indicates a catalyst efficiency problem in the warm-up converter on bank 1.

The OBD2 system uses oxygen sensor readings before and after the catalytic converter to measure catalyst efficiency. 

When warm-up converter performance drops below a certain level, code P0421 will set and the malfunction indicator light (MIL) will illuminate.   

Causes for this code include:

Defective catalytic converter (likely cause), restricted exhaust, exhaust leaks (cracked exhaust manifold).


In most cases, you can easily spot the inactive warm-up converter by looking at oxygen sensor readings after the engine reaches normal operating temperature.  If the post cat O2 sensor voltage reading mirrors or mimics the pre cat sensor, then most likely the converter is dead. 

Check for technical service bulletins before replacing components.  Some manufacturers may have PCM recalibration (software updates).  Some models may also have extended warranties to cover problem converters. 

When replacing catalytic converters, be sure the new converters are certified to do the job.  Some aftermarket converters may not satisfy the O.E. vehicle requirements, causing a return of code P0421, along with some unneeded aggravation.

Root causes:

If converter replacement is necessary, don't forget to address the problems that may have contributed to damage of the original converter.  A rich running or misfiring engine is probably the number once cause of converter damage.  Check the basics.  Ignition coils, wires, plugs.  Look for fuel system problems like bad injectors, evap system faults (fuel saturated charcoal canister), leaky fuel regulator, etc.. 

Did you know?

Did you know that motor oil can cause catalytic converter damage?  It can if it's blended with too much of the anti-wear additive, zinc dialkyldithiophosphate.