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Discussion Starter #1
Today my check engine light came on just after filling the tank with gas. A scan gave me code PO420 Which is: "Catalyst System Below Efficiency Threshold Bank 1".

I then erased the code and then took the truck out for a drive of about three or four miles but the check engine light did not reappear.

Any ideas as to what may be going on?
 

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in the past year (six months apart) i have had the same code twice. both times after putting in suspect MEXICAN gasoline. once from 5 gallon jugs and once from a state owned Pemex station. both times i cleared the code that was that. i don't know what the description means but i'm glad to see someone else has a gas related symptom. you probably got some contaminated/watered down gas..... i wouldn't worry.
 

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Discussion Starter #3
Thanks Baja... that's precisely what I needed to know. It makes sense given the nature of the code.
The code means that the catalytic converter, located between the two bank-1 oxygen sensors, is not performing as well as it should perform. Steam probably has a too high oxygen content.
 

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...The code means that the catalytic converter, located between the two bank 1 oxygen sensors, is not performing as well as it should perform...
Not necessarily. It usually means that the sensor itself is failing. I have yet to see someone post up this code and it turned out it really was a failed catalytic converter.
 

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Discussion Starter #6
I'm not saying that the catalytic converter is failing rather I'm saying that the gases inside the catalytic converter temporarily contain steam because of water contamination in the fuel. Therefore neither the sensors nor the converter are bad. They are doing what they are supposed to do. The oxygen sensors are telling the computer that the fumes inside the catalytic converter contain elements that ain't supposed to be in there... namely steam which is hot water (H2O) the chemical symbol for water and which means 2 parts hydrogen to 1 part oxygen. The oxygen sensor at the South end of the converter sniffs the oxygen in the steam and reports to the computer which in turn activates the check engine light.

Another way of looking at it is that if you have lots of hydrogen and a good deal of oxygen, and if you mixed them together 2 to 1... you could take a bath!
 

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I posted about this a few weeks ago and I too have had my tundra throw that code for the past 5 years every 6 months or so. With a scangauge I just reset it and continued on my way. I know it wasn't the cat because I have had my truck smogged in California twice since it started to throw the code and the emissions were perfect. I know the code points to below threshold emissions but that is what the sensor is reporting, what about the sensor? The code started to appear more often until it came on every other time I drove. I ordered a new rear 02 sensor and replaced my rear O2 sensor and the code has stayed off since proving it was not the cat but the sensor. The OBDII never reported a sensor problem but that is what was causing the code. My rear sensor was over 145K old, can't complain. I have a 2000 V-6 manual turdra and have seen numerous reports of this over the years and most replaced the cat which is serious money, the only reason I never did this is because of my smog tests pointed to another problem.
 

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so does that mean, "don't worry about it?" WHAT is the "sensor?'
The "sensor" is the downstream oxygen sensor. It is an emissions control component installed in modern vehicles due to EPA emissions regulations. It is supposed to measure "catalyst efficiency", i.e. the ability of the catalytic converter to attenuate unburned hydrocarbons. I am of the opinion that this function is tenuous at best, at worst it is an easy money-maker for dealer service bays. They love to scare you with a failed cat story to the tune of $1,400 when the problem is a $90 sensor that anyone with a socket wrench can replace in 15 minutes.

Note that our 4-wire sensors are 'heated' units, i.e. in addition to a data feed it also is supplied power to get it up to operating temperature (~1500F) quickly so the data is valid. This heating element is statistically usually the failing element in these sensor units. When either the heating element fails/burns out or the sensor core is degraded and not able to properly measure the electrical resistance within specs, a CEL code is thrown.

Unlike the upstream fuel-air sensors, a failed/failing oxygen sensor doesn't negatively affect the actual operation of the vehicle's engine, that's why some people feel comfortable just disregarding it when it comes on.
 

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Discussion Starter #10
Remmy... I agree that, of the two, if it is a part failure then it is the sensor and not the cat. But I suspect that Baja and I may have developed the code because we each had run low on gas and may have picked up water condensation from a near empty fuel tank.

If so, the Southern sensor would see the water's oxygen as it passed through the cat and in turn reported a code condition to the computer. I'll keep an eye on things to see what happens in the future.
 

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I got a P0430 a while ago, cleared the code and it did not come back yet (drove about 2K miles since then). Get under the car, using your fist or a rubber mallet to bang on the converter. If it looks ok and you do not hear any rattling inside the converter, it is probably ok. Like some else said it could be bad gas, failing O2 sensor or just a “hick up”.
 

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Remmy... I agree that, of the two, if it is a part failure then it is the sensor and not the cat. But I suspect that Baja and I may have developed the code because we each had run low on gas and may have picked up water condensation from a near empty fuel tank.

If so, the Southern sensor would see the water's oxygen as it passed through the cat and in turn reported a code condition to the computer. I'll keep an eye on things to see what happens in the future.
Earl,

Absolutely. If you can pinpoint an error code to the high probability of poor fuel quality, even better! No doubt you'll run into that type of situation in MX. Any SOTB trips I make (very few anymore), I always made sure to bring a bottle or two of Sta-Bil with me as insurance against degraded gasoline. Our EFI-controlled, emissions-monitored engines are just too sensitive.
 

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i got a p0420 code on my tundra. I switched the rear 02 sensors between the left and right but the code came back on bank 1. It didnt switch sides with the 02 sensors. Does that mean that the sensors arent the problem and i have an exhaust leak or bad cat on bank 1?
 

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Discussion Starter #14
Did you clear the original code? If not it will keep your light on and it will appear as though you are picking up a code caused by the new arrangement. Clear your codes and then try the new arrangement.
 

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No I did clear the code after switching the rear o2 sensors. It took several days and lots of miles for the light to come back on but it was the same code unfortunately.
 

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Discussion Starter #16
I don't think it is a cat or similar problem. I suspect the fuel or water condensation in the tank.
Run your tank low and then refuel at a different gas station and keep clearing the code as you try different things.
 

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I've diagnosed and repaired hundreds of P0420/P0430 codes on Toyotas. Rarely, extremely rarely, is it caused by a bad oxygen sensor. As far as the heater circuit... never. The heater circuit is constantly monitored and there are separate codes for the heater circuit. Diagnostics are pretty difficult to do without the proper equipment, nearly impossible. You need to be able to graph the oxygen sensor (after the cat.) to determine whether or not the cat. is working properly. A vehicle with a properly functioning cat. will have an oxygen sensor voltage of approx. .8 V. A vehicle with a bad cat. will have an oxygen sensor voltage that fluctuates between approx. .2V and .8V in a sin type wave form. On a vehicle with oxygen sensors before the cat. the graph of the pre and post cat. oxygen sensors will be very similar. A vehicle with a pre cat. A/F sensor will have similar looking wave forms but the voltages will be different because A/F sensors usually are 3.3V (3.0V to 3.5V). I put cats. on 3 different Toyota vehicles just this week alone.
Catalytic converters can go bad slowly, setting a code that can be cleared and not return for a while. Water in the gas will not cause a P0420/P0430. You will always get a P0171 (System Too Lean) and most likely misfire codes as well. The air fuel ratio sensor (pre-cat) detects the water in the gas, not the post-cat oxygen sensor which sets the P0420/P0430 codes. A vehicle with water contaminated fuel will most likely have a perfectly good .8V at the oxygen sensor, though sometimes it will be a little low.
Long story short, if you have a P0420/P0430 code, I suggest paying the money to properly diagnose it if you can't do it yourself. It is one of the more common coded that the do-it-yourselfer just can't diagnose without the proper equipment.
 

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I've diagnosed and repaired hundreds of P0420/P0430 codes on Toyotas. Rarely, extremely rarely, is it caused by a bad oxygen sensor. As far as the heater circuit... never...
OK, since you have so much experience after having "diagnosed and repaired hundreds of P0420/P0430 codes", statistically, what is the most prevalent issue that ultimately required "repair"??
 
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