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Dtc Code P0420


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hi all i recently bought a lexus is200 2002 with 75k on it

i had a engine light come up on the dash

scanned it and got dtc code P0420 (catalyst system efficiency below threshold)

i looked at some live data while at idle with engine at operating temp results:

02 sensor 1 bank 1: 0.845 to 0.75 volts.... fuel trim 0 to +8% fluctuating

02 sensor 1 bank 2: 0.60 to 0.760 volts.....fuel trim -1.6 to +1.6 fluctuating

02 sensor 2 bank 1 0 .05 to 0.10 volts fluctuating no fuel trim info

02 sensor 2 bank 2 just said n/a

are these normal results or is sensor 2 bank 1 broken or do i need a new cat??

thanks martin

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I would say new Cat. You cant count on the Sensor after the Cat if the Cat is clogged up.

Edit: You can count on the sensor, but you will get a very different reading from the front ones :)

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The voltage on B2S1 looks low because you are at idle, you need to monitor it when driving to check it is going high and low and to make sure it doesn't give a -ve voltage.

I would check for exhaust leaks and then get an emissions test as it could be a faulty sensor or a faulty cat. If the emissions are high then the cat needs to be replaced. If you have some previous MOT results take a look to see if the levels have been going up over the last couple of years.

You can also try banging the cat, it is rattles then it's broken up inside and will need to be replaced.

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ok lads thanks for the advice.

ill try looking at the data whilst driving but the OBD connection is right above the throttle kinda stupid place to put it

the emissions for the last nct or mot which was only at the start of june were:

at low idle : CO 0.18% (LIMIT IN IRELAND NOT ABOVE 0.50%)


at high idle: lambda 1.01 (LIMIT IN IRELAND not between 0.97 and 1.03)

2500rpm CO 0.02 vol % (LIMIT IN IRELAND ABOVE 0.30%)


these seem like good enough results but i dont know much about emissions, ill jack up the car tonight and see if i can find any leaks

i have since reset the MIL light and driven about 30miles and it hasnt come back on yet

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So are these good enough results to rule out the cat itself?

Also the car seems to be using alot of petrol 20mpg , the idle seems very slightly rough on first start up and it seems to take longer to start every time

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The readings indicate it was doing a good job, shouldn't go bad that quickly. Sounds like the engine is running rich.

I would check for exhaust/manifold leak.

Possibly the pre-cats have broken and started to block the exhaust pipe

Faulty upstream O2 sensor.

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ok so i was looking for info on the 02 sensors when i found this diagram


turns out you can only get a p0420 on b1s2 .

which is also the sensor i originally thought had a low reading so im guessing its probably what threw out the code.

ordering a new one tonight but not sure what one to buy off eBay are all 3 sensors before the cat the same? or are they position specific?

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yes B1S2 produces the error code but it is rarely the sensor that it at fault. It is doing it's job and reporting something isn't right with the emissions. If there was an actual problem with the sensor you would typically get different codes produced.

I believe the IS200 sensors are all standard 4-wire O2 sensors, just position specific in terms of cable length.

If the downstream sensor was faulty you wouldn't see other issues in terms of mpg or idling as it is just there to make sure the cat is working correctly on the IS200, it's results aren't fed back to calibrate the upstream sensors as they are on more modern management systems using wideband air/fuel ratio sensors. You could completely disconnect it and it would have no effect on how the engine performs.

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How likely is it that the pre cats have failed?

I really hope it isn't coz they cost a fortune

Also is it possible to just gut the pre cats and re-weld without the ecu keep throwing codes and pass an emissions test?.... And will the thing distort after welding?

Would replacing the main cat typically solve the p0420 code I got?

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Certainly possible for pre-cats to start breaking up and then clog up further down. :shifty:

Became a problem for TTE supercharged cars so they started swapping manifolds for ones without pre-cats as part of the conversion.

My car doesn't have them and hasn't had emissions problems with the MOT yet. :lookaround:


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