Why only in Cape Cod?

My wife and I own a timeshare in Cape Cod and live in the Philadelphia area. When we drive to Cape Cod, the engine check light comes on. It turns out to be a signal regarding the catalytic converter.

The second time this happened, we asked our mechanic for advice. He told us to go to an auto parts store and purchase a code reader.

So now when we go up to Cape Cod and the light comes on, I reach under the dash, plug in the reader, clear the code (0420), and we go on our merry way, saving the world’s supply of electrical tape.

My question is “why only in Cape Cod?” It doesn’t come on during other long drives. The light comes back on during short stop & go trips on Cape Cod.

I suppose we could go to the expense of replacing the converter but for one or two weeks a year, it doesn’t seem warranted.

Any explanations?

It’s seldom the converter is at fault, so don’t replace it. Year and miles?

do you go to the same gas station every time you go to Cape Cod? Maybe you’re getting crappy gas every time you go.

Does it go on while going over the bridge, or after you have been on the cape for a while?

Um, Cape Cod would be at or near sea level would it not? Would you get the same symptom if you picked another point along the coast? So what measures barometric pressure and makes the lean/rich adjustments accordingly? I’d be looking at maybe a tired mass air flow sensor or the air flow sensor. One is cheap, one is expensive.

Thanks for all the replies. The Toyota Sienna has about 140K miles and is a 2001. Most (but not all the time), the light comes on after we cross the bridge to Cape Cod. We get gas from a variety of places along Cape Cod.

If the problem becomes worse, I’ll check out the sensors.

Now for two laughs for you all.

  1. Toyota has a service bulletin that recommends replacing a computer board instead of the catalytic converter. The computer board costs about what the converter costs. I have been told by a friend that the reason why is that later model Siennas have a different computer that can be reprogrammed. For economic reasons, Toyota never set up the facility to reprogram the computer in our Sienna.

  2. The first time I had an engine light on our Sienna, it was because the gas cap was not tightened sufficiently. The Toyota dealer told me the nature of the code and offered to reset it for us for $90. I asked it if it would go out on its own in 500-1,000 miles. He gulped and said yes. I told him that $90 could buy a lot of electrical tape to cover the light until then. The funny part was that in the meanwhile, the mechanic cleared the code anyway. No $90 out of my wallet. Ironically, my code scanner only cost $50.

Happy motoring.

Not sure why. If you don’t want to sound like you’ve arrived at the Cape from PA, you might want to take up the habit of saying “ON THE Cape”, not “IN Cape Cod”. Just Saying.

It is not important to figure out why the check engine light comes on only when you go to the Cape. What is important is to start with the code the light corresponds too and find out what is actually wrong with the car. In other words don’t waste energy with this “side issue” just make in into a funny little story to tag along the not so funny story of havig to spend money to fix your car.

When customers present me their car and the complaint is tied to some odd incident such as the one your present I at times ask which issue do the want me to work on first, the one that matters or the one that makes a good story.

Are you serious?!? If the problem only happens under certain circumstances and every time those circumstances are met, you don’t feel that is relevant information to use in your diagnosis?

It’s not an odd incident if it happens repeatedly under the same set of circumstances. Some mechanics, perhaps most, would use that information to their advantage in expediting a resolution to the problem. Gee, what’s different about the Cape versus rural PA? Could it be the relative difference in atmospheric density or moisture content in the air, salt spray or perhaps the gasoline is a different composition there? At least consider the possibilities related to the change. Others can’t seem to be bothered with those pesky details…

The part that makes the CEL come on is the rearward oxygen sensor…They have a design life of 100K miles. When they get tired, they throw the “Catalyst efficiancy below threshold” code."

I had the same problem and a new oxygen sensor cured it…These sensors sell for $69-$89 each and can be changed in 10 minutes with a common 22mm wrench…

I would guess Massachusetts has some stupid gas law like California . Bad gas is your answer .

If it was bad gas wouldn’t everyone’s ck. eng. light be going off?? Including the locals especially?? They would be up in arms about it. I have my doubts that bad gas is the answer.

Bad gas gets blamed for all manner of problems. Your logic is impeccable. If it wree bad gas everyone in massachusetts would be having problems.

The code being stored suggests low efficieny in the cat converter for bank 2. What triggers the code is the bank 2 oxygen sensor downstream from the converter. The ECU compares its signal to the signal from the upstream sensor to see if there’s sufficient change in the signal to indicate that the converter is working. If the signal is marginal, it trips the code.

The problem is that the sensor itself could be bad or the converter actually be inefficient. This can easily be determined by looking at the signals from the upstream and downstream sensors on a scope. A scope will show the actual traces and be very definitive.

It is posiible that after 140,000 miles the converter is actually becoming marginally efficient. It works by allowing the exhaust gas to flow past and contact a honeycombed ceramic coated with Platinum-palladium (a rhodium). This metallic coating when hot, wekens the bodn between all the NOx molecules that come in contact with it and converte them to seperate nitrogen and oxygen atoms. The carbon monoxide and the unburned hydrocarbons can then complete their combustion, now having free oxygen to work with, and emissions are reduced.

Over the years, however, the catalytic coating can become coated with exhaust byproducts. That weakens its ability to directly contact the NOx molecules and reduce its efficiency.

So you coud have a bad downstream oxyygen sensor or an actual inefficient converter. And there is a way to tell. However, many people consider it cheaper to simply change the downstream sensor and if that doesn’t work then change the converter. The sensors do have a way of going bad over time. They too rely on actual contact and can become coated.

Some good advise in the thread so far.

Personally, if it were my car, I would replace both oxygen sensors, since they now have 140k miles on them, and their lifespan is typically only 100k miles.

Also, while it probably isn’t “bad” gas, per say, it might actually be caused by a different blend of gas, which either has more or less oxygen in it’s formula in comparison to what you have back home, combined with your converter/sensors starting to weaken.

The formula change, and the higher temperatures from driving so long probably combine to drive up the emissions that the rear sensor is seeing, which then triggers the light to come on. Probably more stop and go driving on that trip than on your other trips.

Have fun!