Strange 'check engine light' situation

Hoping you guys can help me out with this one. My wife’s 2005 Sienna is a very reliable vehicle. But occasionally when I drive it a fair distance–50 miles or so, but not always that far–the check engine light will come on. (It’s something vague about emissions, but isn’t it always?) And then a day or so after my wife drives it again, the light goes off. The van seems to run fine at all times. So is it something I’m doing when I drive it?

You have to get the codes read. Even if the light is off, the code is still stored. It might be as simple of a loose gas cap.

In most states except CA, part stores would read the codes for free.

Who was vague about it? Did someone pull codes and provide what you felt was a hazy answer as to the cause?

Yes, it is your driving. The van hates you.

Seriously, tho. Different people have different driving styles. Your driving style is obviously triggering a condition that the van’s computer sees as a problem. Your wife’s driving style does not. I can tell you that the condition will worsen to the point where it will trigger the light with her later, because they always do. So can you get the code read an post it here? It will be in threform of Pxxxx.

I did have the codes read. That’s how I knew it was something vague about emissions. But knowing that it’s important to know WHAT the codes are, I dug around and found the printout from when I had them read. The codes are P0420 and P0430 - Catalyst efficiency below normal, banks 1 & 2. Does that help?

Do a google search for “P0420 and P0430”, you will get lots of info. They imply that your two catalytic converters are bad, although it could be the sensors.

For example:

Yes, I understand the implication. But as I said, the van runs very well… none of the signs of cat failure that I have experienced in the past. And it still doesn’t explain why only I am getting the check engine light. Are the sensors that particular?

The oxygen sensors–which are more likely than the catalytic converter to be the source of the problem–could be reacting differently to two very different driving styles.

Does the OP accelerate more aggressively than his wife?
Does he drive faster than his wife does?

On the contrary. I may eventually drive faster than she does, but I tend to accelerate more smoothly to get there… pedal stomping in a minivan just doesn’t appeal to me. But I do see how the O2 sensors could be reacting differently to that behavior. Is this something that could/would be resolved with new sensors?

There are many codes that can be thrown and the vehicle will still be running fine. The codes are not just for engine performance…but also for emissions. So the vehicle may be running fine…but it’s not meeting the EPA emission standards.

Yes, this could possibly be a case of needing new sensors. Whatever you do, don’t replace the catalytic converter except as a last resort. More likely the sensors are at fault, and they’re a lot less expensive to replace.

The rear oxygen sensors are what is throwing the code…If you do anything, replace those first. They have a design life of about 100K miles…This USUALLY cures the “problem” which is not really a problem depending on how “Green” you are…It has no impact on the serviceability or safety of the vehicle…

SOMETIMES the sensors are located (behind the CAT) in a location where you can get to them and change them yourself, saving a fair amount of money over what a shop will want to replace them…

Certain tests of emission components can only be performed by the car’s ECU (computer) under the right conditions. Some of these tests require a steady cruise at highway speeds. This is why the MIL (Check Engine light) only comes on during these conditions. I expect the test is accurate and you either need a new catalytic converter or new oxygen sensors.

There’s an O2 sensor before and after the cat. The computer compares the readings. They should be different, right? B/c if they were the same that would mean the cat isn’t doing anything. While it is possible other things could cause it – faulty sensors or wiring or the computer is going crazy for some reason – mostly likely the cat is nearing the end of its life. As mentioned above, the difference in driving styles and/or the places your are driving could be the reason it happens to you, and not to her. The readings from the O2 sensor will differ in different situations. The reason is the O2 sensor is in the computer’s feedback loop that determines how much gasoline needs to be injected at every second of the trip. The sensor alternates between reading rich and lean. At idle it will be alternating one way, while at 60 mph on the freeway, another. And going uphill, still another. It must be that certain ways are more sensitive to the computer discovering that the sensor readings before and after the cat are too much alike, and, alas, the cat is caput.

Let me guess, your Sienna has about 150,000 miles? That’s about when Toyota cat converters seem to become inefficient to the point of intermittently tripping the code.

By all means try changing the downstream oxygen sensor. But prepare to change the cat converter in the not too distant future.

I agree with Jesmed1. Do not think of replacing the cat converter until you have replaced the relevant sensors and drive the car for a while.

On Sienna chat, various people have bought OEM sensors made by Denso from rock autoo and have reported them as working well in most cases. Denso makes parts for Toyota. My alternator on my 2002 Sienna is a Denso part.

Over the years here on Cartalk forum many mechanics have said they seldom or never replace the converter for P0420, the sensors seem to always fix it.

Others say they always have to replace the converter. However, when I ask, they admit, sure they do replace the sensors while they are at it. Think!!!

I have had intermittent P0420 on my 2002 Sienna. It comes and it goes. Just last week, I bought the last two of three relevant Denso sensors from Rockauto and my clever son-in-law put them on. I won’t really know if they fix it for some time, since they have been intermittent and seldom fail. Since labor was free, total cost for Bank 2 Sensor 1 and the output sensor was less than $200.

He hooked up his new Innova mapping scanner, around $290 at Amazon, and the two sensors, the first one ran well over 3 volts, and the output one ran well under 1 volt, though it would climb up and down.

On Sienna chat, people have reported very poor results on Siennas with other brand parts. I have no financial ties anywhere; I am only reporting what I know and have read as well.

How P0420 works, the computer looks at the sensors before the converter, and compares it to the sensors after the converter. if the converter is working correctly, there will be a difference. It there is no difference, it tells you the converter is bad. Usually not so. Converters start to fail, in my experience, around 100,000 miles I don’t mean they all fail. I mean the first ones to fail will fail at that point, while others run much longer.

My 2002 Sienna is called First Generation. Yours is second generation. On mine, the sensor on the output of the converter is plugged in, under the front passenger seat. My SIL took the 4 nuts or bolts, I forget, off the passenger seat and unplugged the cable, then shoved it out the bottom, reversing this for the new sensor.

It had been replaced before, so he was able to break the mounting bolts loose and reuse them with anti-seize.

Haynes recommends disconnecting the battery to disable the air bags before working on this. If you do, be sure you have the activation codes on all your electronics and alarm before you do.

Scottie Kilmer has said if you can get the converter off, toss it in a bucket of water with dish soap over night to clean it. And, a Cartalk poster not long ago said he had done so, and little white pieces came floating out.

Someone said the converters wear out. Converter experts deny this. There is a fine screen in there of horribly expensive metal, and the very word catalytic means it does not itself change in operation. If it stops working, it is because the screen is clogged with crud, the screen does not get consumed with time. That is why soaking it in dishwater can clean it.

Another cause of failure would involve extreme damage to the converter, of course.

Catalytic converters can and do wear out, even under perfect operating conditions

Maybe not for every car, but it does happen

I am not saying that is the case here, but it is a possibility

I am not making any judgements as to what the car actually needs, because me and my tools are not there, to diagnose it

Nobody has checked any pids

Nobody has checked if there are any exhaust leaks, which could be affecting sensor readings

Nobody has checked backpressure

There has been no diagnosis of any kind performed, at this point

Retrieving fault codes is just that. Nothing more and nothing less

Perhaps we are disagreeing over a difference in terminology. By its nature, catalytic converters cannot and do not “wear out”. That is what the word catalytic means.

They can become clogged over time, or coated with crud which makes the screen unusable. That in the end acts exactly how it would be if they did wear out. Stops working.

In the end, it doesn’t matter much since the result is the same, I suppose. I continue to say no one should replace an expensive converter until he has replaced the relevant sensors and driven it for a while.

And, if a mechanic has the converter first on the list of parts to replace, simply because the code says cat is bad (low efficiency) find another mechanic.


I agree that the sensors are sometimes the reason for the light

But there are situations where those get replaced, and the customer is back soon after. Because he needs the cat

I say soon after, because the cat monitor needs certain conditions to run

If you replace the sensors, at least let the cat monitor run to completion and pass before telling the customer their car is fixed

And might be a good idea to look at mode 6 data, so that you can see exactly how it passed. With flying colors or barely. If barely, it’s quite possible the cat is very near the end of its useful life, and the customer will soon be back

of course, it’s also a good idea to also do all of those tests I mentioned earlier