Breaking the law
(This carries a massive fine from the EPA, don’t do it...)
A good way to understand P0420s is to understand how criminals trick the PCM into believing the catalytic converter is efficient, even when it’s not there! Simply add oxygen sensor extenders, tricking the post-catalytic converter oxygen sensor into believing the air it is sensing is cleaner than it really is. So, a P0420 has nothing to do with the cat and everything to do with what the post-cat oxygen sensor thinks is happening. Obviously, a rear O2 that is in a vehicle that abides by federal and state regulations will only be affected by the catalytic converter or exhaust leaks. The PCM simply presumes that the rear O2 sensor data is sufficient in determining how efficient the cat itself really is.
The comeback-proof approach to P0420s
Whether the vehicle you are dealing with is a Toyota or any other manufacturer, the way you approach diagnosing a P0420s is exactly the same after you plug in your scan tool.
1. Check for other DTCs. Obviously if the vehicle has a misfire, system lean/rich, MAF or any other DTC that indicates that a condition exists that prematurely breaks down a catalytic converter you will need to fix that DTC first. You need to diagnose and fix these DTCs before you proceed.
2. Always check fuel trim. If you cannot recall what the LTFT (Long Term Fuel Trim) was before you change that catalytic converter, chances are your diagnostic approach has more holes in it than Swiss cheese. Only look at the other PIDs if fuel trim is excessive (plus or minus six or so). If fuel trim is good, you can proceed.
3. Check TSBs. Always do exactly what the TSB tells you to do. The manufacturer spends billions on research and development. They are neither stupid nor intent upon wasting their time giving directions on how to repair specific problems their vehicles pose. If they tell you to do something, do it!
a. Burn-off procedures. Fuel system cleanings and burn off procedures performed as a “cheap P0420 fix” are a load of you-know-what. I have never, ever seen a fuel system cleaning or burn-off procedure resurrect a catalytic converter on a vehicle other than a Toyota. Not so coincidently, Toyota is the only manufacturer that I am aware of that recommends this as a P0420 repair. I have had a 50% success rate fixing Toyotas this way.
NOTE: I have killed two catalytic converters by doing a simple fuel system cleaning as preventative maintenance. Both vehicles had over 70,000 miles without having ever had a fuel system maintenance service, and I believe large pieces of carbon and high temperatures trashed the catalytic converters. Granted, one vehicle was a Nissan and the other a Subaru...who knows how close either converter was to being on its way out. But, be careful overselling fuel system cleanings or you will actually be doing your customer a disservice.
b. Reflashes. Who decides what makes a catalytic converter “inefficient.” Well, I don’t and neither do you. The software in the PCM does. So, in a way the PCM is the referee that discerns whether the catalytic converter is off-sides or not.
Now, I know for a fact that there are certain cars that even with the whole substrate punched out of the cat, they will never throw a P0420! Why? The software in the PCM is looking the other way. I believe that the manufacturers make software updates that make the referee look the other way when it comes to a catalytic converter failing to do its job.
So, how does this information help us? Even if a TSB asks you to replace the catalytic converter, it’s possible you can get away with just a reflash. This is very common on Nissans and Toyotas. If there is a PCM software update available, you can sometimes get away with aftermarket catalytic converters when you otherwise couldn’t. Why? The software update pretty much bribes the referee to rule in the manufacturer’s favor. “The cat’s fine, there’s nothing to see here, move on...”
4. Test the rear oxygen sensor. Look at how the rear oxygen sensor is working by graphing the PID. Does it react from you punching the throttle? Can you shift it rich or lean by adding propane or causing a vacuum leak? These reactions indicate a good rear oxygen sensor. However, an oxygen sensor whose signal just dies or is stuck rich or lean might be throwing a P0420 by tricking the PCM into thinking that the catalytic converter is inefficient. So, make sure your rear oxygen sensor is good before condemning the cat.
5. Check for exhaust leaks. This is important to do in order to make sure that nothing is throwing off the oxygen sensors, especially the rear O2 sensor. Fix these before condemning the catalytic converter.
An exhaust leak in front of a sensor can throw of its readings, which in turn will make the PCM believe that a catalytic converter is in worse condition than it really is.