There are several reasons a P0420 will come up as an error and illuminate the MIL. A lot of people are scared of it. They get all excited and search all over the internet to try and guess at what they should do. Reading good advise and bad trying to save money and hoping its not a Catalytic converter gone bad.
Well, I have been trying to inform as many people as I can that the 420 code is not a scary animal that is going to eat your wallet. This code simply means that somewhere in the chain of events that controls the overall performance of the engine there is a problem and you have to find it.
List of possibilities:
Fuel quality (low quality fuel a/k/a Murphy Gas)
Fuel quantity (filter or pump issue)
ignition issues (plugs, wires, coils injector flow)
temperature control (too high or too low)
exhaust leaks (not only an annoying noise, but it effects proper value reporting to the ECM by the AF and O2 sensors)
poor amperage conductivity (i.e. battery or cabling)
Sensor issues can be:
MAF (Mass Air Flow Meter)
MAP (Manifold Absolute Pressure)
ECT (Engine Coolant Temperature)
AF (Air/Fuel ratio a.k.a. Front O2 which can be a single bank 1 for 4 cyl or bank 1 or bank 2 H6)
O2 (This is the actual O2 behind the CAT that lets the computer know how the CAT is functioning)
So, in an attempt to show some data and explain simply what I found, I decided to video some diag segments and show the live data from the vehicle. I will do this to show differences as they come up. Today I had 2 Subarus with emission issues. The first was a 96 Legacy wagon AWD 2.2 that was low on coolant due to a crack in the radiator and the ECT value was causing issue with starting because it controls the IAC position. It reported high temperature to the ECM which closed the IAC. No air - No start. The second is the following.
I am uploading video now. It is from a 2001 LLBean with just shy of 199,000 miles. The MIL was on for a week and went off this morning as the owner was driving it to me. I checked all the systems on the car for error codes and the only one was a P0420 still stored as active. So I began watching data from the car.
I first let it sit for a couple hours to cool down. Checked the oil, dirty but full. Coolant level was good. Battery amperage checked out excellent at 705 A output and conductivity measured 650 A at the block so grounding was good. Air filter was mediocre so I left it for now to keep the car in the same condition it was in when the MIL set. The owner hadn't driven it much since the light, so the same gas was in the tank.
There is a point in the video where I thought one thing and said another. I referred to the oxygen sensor as storing oxygen when in fact I meant the sensor was reflecting the cat storing oxygen then it let go, the cat. There still may be an issue with the cat, but at this time I cannot condemn a part that is working through another issue with the engine.
In this case, the engine is overheating due to a stuck thermostat and the HC's are actually being burned due to overheating before reaching the CAT. Since this is occurring and the ECM is programmed to expect other values from the O2 sensor, it reports that the CAT is not functioning as it was designed and illuminates the MIL. So a thermostat is in order as well as a fuel filter which you will see why toward the end in the fuel trim value for bank 2. Since the engine runs almost perfect up to the point of overtemp, there is no need to replace the ignition plugs at this time. It was recommended based on the mileage as preventive maintenance. The owner is considering selling the vehicle since she doesn't drive it much anymore and has a smaller car she uses for the higher MPG's, so the car is parked until she decides to repair it or its sold.
2001 Subaru Outback LLBean P0420 Diag - YouTube
You also need to know that if any of the above issues go on too long, it will damage the catalytic converter to the point it will necessitate replacement.