Catalyst System Efficiency Below Threshold

This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model. So this engine code article applies to Nissan, Toyota, Chevrolet, Ford, Honda, GMC, Subaru, VW, etc. P0420/P0430  is one of the more common trouble codes that we see. 

The main symptom to the driver is the illumination of the MIL (Check Engine lamp). You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle.

A code P0420/P0430  may mean that one or more of the following has happened: 

• Leaded fuel was used where unleaded was called for (unlikely) 
• A damaged or failed oxygen / O2 sensor 
• Downstream oxygen sensor (HO2S) wiring damaged or connected improperly 
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe 
• Failed or underperforming catalytic converter (likely) 
• Retarded spark timing 
• The oxygen sensors in front and behind the converter are reporting too similar of readings 
• Leaking fuel injector or high fuel pressure 
• Cylinder misfire 
• Oil contamination

Some suggested steps for troubleshooting and fixing a P0420/P0430 error code include: 

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. 
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). 
• Inspect the downstream heated oxygen sensor, replace if necessary 
• Replace the catalytic converter.

You can look at the temperature of the exhaust just before and just after the converter using an infrared temperature gun. The output should be about 100 degrees Fahrenheit hotter when the engine is fully warmed up. 

Overall probably the biggest mistake vehicles owners make when they have a P0420/P0430 code is to simply replace an oxygen sensor (02 sensor). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily. 

Many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.