P0420, P0430, P0421 Catalyst Efficiency Codes

So many techs fear these group of codes because they have been bitten before by them. In my opinion, these are some of the easier codes to solve if you attack it systematically. The first step is to know the enemy. The general code set criteria is as follows; When the the downstream oxygen sensor is within 80% of the activity of the upstream oxygens sensor during a catalyst test the code is set. The key to this is graphing our scan data. Years ago scan data was much too slow to trust graphing it. But, todays datastreams and professional scantools make this a trustworthy venture. Here is a snapshot of`data that was graphed.

In the above we have the following pids graphed. Engine speed, upstream oxygen sensor, downstream oxygen sensor, and throttle position voltage. As you can see we are raced up at 2514 rpm. Is this catalytic converter doing a good job right now? No, not really at all. What I stress here is look at the symmetry between upstream and downstream sensors. They are almost identical. Are we within 80% activity? Looks like it. Now, before you plunk that money down for a catalytic converter there is a couple of things to do. I always take the car for a thorough test drive and recheck oxygen sensor activity out on the road. There are many converters that need a hard test drive to verify operation or "light off". The other test procedure I do is either introduce carb spray or propane into the system to make sure both upstream and downstream oxygen sensors go full rich. Sometimes, I can get away with just "flashing" the throttle a couple of times and checking. Confirm that the vehicle is at operating temperature-bad thermostats,  radiator flow, or improper fan operation can play havoc with catalyst operation. The final item to check is fuel trim. Make sure your fuel trims are tight. Fuel trims that are elevated either way will cause catalyst issues.

As you can see in the above snapshot our fuel trims are excellent. Always check fuel trims at an idle, raced up 2500 rpm no load, and at cruise steady throttle out on the road. There is usually a reason behind catalytic converter failure. The reasons can range from a converter that has been digesting misfires, bad fuel trims, and oil contamination. I see the oil contamination issue quite a bit on those pre cat (where the cat is part of the exhaust manifold or directly after the exhaust manifold) vehicles. If you have any of these issues the new converter will go belly up in short order.  So now you are ready to order the catalytic converter. Not yet, definitely run a tsb (technical service bulletin) search to see if there are updated parts or reflashing pertaining to your issue. Now you are ready to get prices on the catalytic converter. A call to the OE dealer gives you a price of $842.31-geez the car is only worth $2000.00. A call to the parts house or a converter supplier and the price is $231.28. Why the big difference? First off make sure the aftermarket one is a direct fit. Nothing like giving the cheaper price only to find out you need the $95.00 adaptor kit and have to be  Michaelangelo with the welder to get this to fit. The second is make sure that the converter you are getting is OBD2 C.A.R.B (California Air Research Board) certified and they can supply the paperwork supporting this. I have seen too many times the shop sells the cheap converter only to have me come back multiple times and the converter to be exchanged two times before it is realized this cheap converter is in fact cheap. I have also seen where the manufacturer will hide behind that the fuel trim is off, the vehicle is not reaching operating temperature, the oxygen sensors are incorrect when there is a problem. This is why I always document everything. There are certain vehicles I do not recommend aftermarket catalytic converters on period. They are 2001 and up GM cars, Honda vehicles, and Subaru vehicles. It seems the aftermarket cannot make the proper converter for these applications, you get what you pay for. 
Another item to be wary of is exhaust leaks ahead of the catalytic converter on codes that never seem to be fixed. An exhaust leak ahead of the converter no matter small will introduce outside oxygen into the exhaust stream. Causing problems and ruin your day. A smoke machine can be used to find small leaks. So what if you have a A/F ratio sensor upstream instead of a conventional sensor. Just look at the downstream sensor graphed and use the hints I have given to diagnose these cars. Most techs think that the catalyst monitor is run during steady highway speeds. That is true of most cars. But, late model GM vehicles will run it at an idle after a highway run. Know your enemy, check your drive cycle routines to determine when it looks at converter status. I hope this helps the next time you have one of these in your bay.