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Buying a scope or scanner for sensors, 2002 Sienna, 192,000 miles, and others

May 2011, there was a posting on the dreaded P0420 bad cat efficiency issue. I reported that my 2002 Sienna had popped that code after running down hill 17 miles on engine braking, on the cuota to Orizaba. There was a lot of discussion on the recommendation by Scotty, Houston radio car talk host, to put chemicals in your gas tank.

Later, my code went away. Several people said it would soon be back.

Last week, around 17,000 miles later, I got the P0420 code on the same trip, but before I entered the high speed highway. I erased the code, expecting it to come right back on, but so far, nothing, and I did the 17 mile down hill run, but not with engine braking.

Someone recently said on a posting that almost always the P0420 code is a bad cat. But, in the past, when I have Googled extensively, most mechanics insist it is usually bad sensors.

The way I see it, it’s one of several things.

  1. Bad sensor(s).

  2. Engine pumping something messing up the cat, that eventually burns off. It does not seem to use much oil.

  3. Or, bad cat. Because it seems to come and go, bad cat seems less than sure.

I would rather identify the real problem, than toss expensive parts at it. Which is the point of this posting.

I bought what I thought was a good scanner a year or two ago. I got an EQUUS 3140, partly so I can run older cars so often found here in Mexico.

It promises freeze frame; and live sensor data. But, sensor data is digital, and bounces all over the place, making it nearly useless for telling me if a sensor is good. Unless someone can give me ideas how to better use it.

Now, people have posted one needs to look at sensor output with a scope. So, how do I do that? Does this mean a new scanner with built-in sensor scope, or a separate scope, and if so how does one hook it up to the car? I think I can pay for a pretty good scanner/scope if I avoid one needless cat replacement, and then have it forever.

Any help appreciated.

When looking at the live data for the O2 sensors, the upsteam or before the catalytic converter O2 sensor signal should bounce up and down rapidly because this represents the cross-counts of the O2 sensor. This is because there’s oxygen present in the exhaust gas. The downstream O2 sensor or after the catalytic converter should be flat lined because any oxygen in the exhaust gas should have been consumed during the catalyst process.


In your case I believe it is a software issue. There are some driving conditions in which the catalyst monitor should be disabled.

There is a service bulletin to replace the Engine control module. However being out of warranty clearing the fault in the rare case that this fault is set would be more practical.

@irlandes a decent scope usually costs thousands. And it takes practice to really become proficient using it. Save the money for a rainy day.

Can you use that scanner to check the ECM part number?

If you have one of the “previous part number” ECMs, I would seriously consider inquiring as to the price of that updated ECM.

Certain conditions must be met for the catalytic converter monitor to run. It’s possible you were meeting those conditions only rarely. That would explain why the code set infrequently.

I have read the conditions in the past, and I bet I meet them often. But, I will review them.

Thanks, for the bulletin information, Nevada. I never heard of this before.

Apologies I posted this with the intention of checking daily for answers, then got busy with a major genealogy project and forgot it. I woke up in the night, and it popped into my mind. Sorry.

Actually, db, in 2011, it was on most of the time when I drove any distance.

That does agree with Nevada’s bulletin, that it came on, then after reset stayed off even after a long trip.

Comment on scopes. Perhaps automotive scopes cost thousands, but I worked at a major electronic factory, contract defense plant, and I assure you good scopes do not need to cost thousands. I retired 15 years ago, but at that time we had usable scopes capable of being calibrated for well under a thousand dollars which did our military tests just fine.

If you Google for Amazon oscilloscopes you can find storage scopes for under $500. Part of a scope is having experience in using them. An experienced person can use cheaper scopes with effect. Those with less experience need much more automated equipment, of course.

Sure, if you want a dual channel Fluke Industrial grade hand held you can spend thousands, but they are not necessary. We never used anything like that; it wasn’t necessary.

Some of our expensive equipment would have been an HP Spectrum Analyzer, though things like that were only needed for very complicated products.

@irlandes I gave you advice about an automotive tool for automotive use.

As far as that cat code goes, I would spend the money on that new ECM, which might fix your problem, rather than that Amazon scope, which might not be used often.

But that’s just my opinion.

If this only happens on a long downhill run, ignore it! Perhaps your cat simply cools off (without any fuel to burn) and this unusual condition triggers the CEL/code…Half the vehicles in Mexico display a CEL, that’s how they ended up in Mexico! Join the club!

I just remembered my son-in-law has oscilloscopes I can borrow. So, how does one connect the scope to the car to see the sensors? Can one do it via OBD-II or must you get down to the sensor, which might be hot?

In fact, if I can figure out how to do it, he’d probably help me. He fixes late model tube stereos, and I taught him how to work on them, being from the tube generation. He is one of those men that you have fun any time you work with him.

You want to back probe the O2 sensor connector at the signal out wire. I use a stick pin for this. Then ground the oscilloscope somewhere on the engine.


Okay, got it. No cool choice. Thanks I will be evaluating all suggestions and looking for part number of PCM. As I get time, of course. I may print this page and the service bulletin and add to my shop manual. Good stuff.