Understanding and diagnosing Toyota P0420 DTC

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Understanding and diagnosing Toyota P0420 DTC

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE A6 certified with a M.A. from Columbia University. (Vehicles were diagnosed by Kevin Quinlan and Alex Portillo. Contributions made by Gerry Truglia.)

Out of all of the different DTCs I see, P0420 is probably the most common money-making job I get. And, out of all the manufacturers I work on, I see P0420s on Toyotas the most.

There is more to this DTC than merely changing the catalytic converter and two oxygen sensors. We have to be very familiar with fuel trim, Mode 6, interpreting oxygen sensor feedback and reflashing to diagnose these DTCs. Before I get into Toyota specifics, we are going to lay some ground rules that will help you diagnose a P0420 on everything from a Toyota to a BMW.

P0420 theory

P0420 equals changing the catalytic converter, right? Not always.

I find it useful to believe in a noble lie that catalytic converters will never go bad unless something makes the cat prematurely degrade. The reason this is a “noble lie” is because in reality many Nissans, Volkswagens, Subarus and Toyotas have catalytic converters simply made with a lesser lifespan than that found on many other manufacturers’ vehicles. However, when diagnosing a P0420, pretend that reality does not exist and temporarily believe in the lie: catalytic converters will never go bad unless something prematurely degrades the cat.

So, your approach to diagnosing a P0420 should be in reality fixing bad MAFs, exhaust leaks, ignition coils, vacuum leaks, and other conditions that create extreme fuel trim conditions that degrade catalytic converters. If you go down the list and find that the vehicle is maintained properly and no condition exists that screws up the fuel trim, only then can you condemn the cat.

Sure, sometimes catalytic converters just go bad. But just pretend that is not true when you are diagnosing that P0420 until you are confident the vehicle just needs a cat.

Understanding oxygen sensors and P0420s

Before the catalytic converter, the oxygen sensor’s voltage will zigzag up and down when graphed. To the contrary, the air-fuel sensor will be at a stable voltage. The post-cat oxygen sensor will be a straight line of voltage if the catalytic converter is good in most cases.

Post-cat oxygen sensors, when good, are at a steady voltage, usually between 500 to 700 mV. If it zigzags, mirroring the pre-cat oxygen sensor, the catalytic converter is highly suspect. Sometimes the post-cat oxygen sensor will have a gap of time between the pre-cat sensor switching voltage and itself. This is often normal during a sudden fuel event that the catalytic converter, even when good, cannot instantly clean up.


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.




Business is business

2001 Toyota Celica P0420: This Celica came in and it was the customer’s daughter’s car, so everything had to be taken care of and just right. Freeze frame showed that the failure happened at about 65 mph, so we knew that the cat only failed when the engine was really at work.

What stood out to us most was the O2 sensor reading when we were looking at the PIDs. The Bank 1 O2 sensor looked suspicious because it did not change when propane was added to the vehicle.

We then looked at Mode 6. Mode 6 showed us the catalytic converter was not presently failing, with a value of 0.253. Unless your scan tool reads Mode 6 continuously and decodes the TIDs and CIDs, it’s going to be a very time-consuming process you will likely skip. Nonetheless, it was testing really close to the limit so we boroscoped the cat after diagnosing but before replacing the bad oxygen sensor to check its physical condition. We saw no obvious problems. Now, what do we do?

We decided to do a Catalytic Efficiency Test using the EScan software. It sounds like we are doing a commercial for ATS, but this is what we really used at our shop before we understood how to interpret the post-cat oxygen sensor.

We did not even have to connect any fancy transducers or anything. Right from the DLC, the EScan compares the oxygen sensors to each other to give a percentage of good catalytic converter health, color coding it so even a service tech like me can understand what he’s reading.

Generally, a Cat Efficiency reading below 70% sets a DTC, but at idle the cat was testing fine. So, we decided to recreate freeze frame conditions at a high engine load and the following occurred. The Cat failed when the engine was working hard, confirming what the freeze frame data told us initially.

We figured that it was time to replace the catalytic converter, but remembering a P0420 class that was presented by my father, we decided to experiment with a Run Rite fuel system cleaning and see if it would act as a cylinder burn-off recommended in Toyota TSBs. I was afraid that this would be the snake-oil part of the fix, but I trusted my father and called the customer. The customer was great and said yes to everything the vehicle needed (including the brakes and a transmission flush, too.)

1998 Toyota Rav4: This vehicle put the “noble lie” (that catalytic converters are good for a lifetime but vehicle conditions make them go bad), to the test. The customer was really “frugal” and had a Rav4 for 180,000 miles without ever doing a tune-up. Now, he had a P0420.

The cat is totally shot, right? Wrong! After new spark plugs the minor misfire was gone, so we did a fuel system cleaning to burn off the cat. Fuel trim was otherwise good and with prompting from the customer we even current ramped the fuel pump just to put his mind at ease. We thought the vehicle was good to go and confirmed this by looking at Mode 6 on the Toyota Techstream.

Summary on diagnosing Toyota P0420s

Toyotas are not much different from other vehicles with P0420s other than the fact that their catalytic converters go bad a lot more often. MAF sensor failures are very common on Toyota Corollas, and you need to keep in mind that reflashes and burn-off procedures are common fixes for Toyotas which might save you a catalytic converter. Otherwise, approach diagnosing Toyota P0420s just as you would attack any other manufacturer and you will be fine.    ‚óŹ

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