P0420 Code

ChumpusRex · July 27, 2015

Hi guys,

I've been getting this code intermittently, and I've got 150k miles, so it's likely that the cats are nearly dead, but I'm just wondering on the best way to deal with this, and what the likely cost would be.

The original O2 sensors are still in place, but the voltages and stuff all look legit, so they probably are still working.

I think the code refers to the main catalyst which is the one in the exhaust header - so, I'm thinking that this could be very expensive.

Any ideas?

toffee_pie · July 27, 2015

newbie has this done, looks like denso sensors needed?

he will chip in i am sure.

ColinBarber · July 28, 2015

It's likely to be the upstream sensor(s) fooling the downstream sensor that there is a problem.

is200 Newbie · July 28, 2015

.... heres my chip in

The error code is different to the one I had for the sensors I replaced.- Lexus do recommend they are replaced between 70 and 80k I was told so yours being original on 150k are doing very well.

There is a noticeable difference with the running and fuel efficiency when replacing the old sensors but do in pairs. Bank 1 and 2 (sensor 1's) pre cat. The post cat sensors I was told should last the life of the car... I would like to see that, but define the life of the car - most cars at your mileage wound be pushing up daisys but these are very robust cars so I would not consider that to be a bad mileage as long as its looked after.

You can also check your exhaust for holes / blowing - the sensor may register this and throw up an error but I would expect to see problems with the exhaust between the sensors for this to happen. Possible, as theres quite a lot of pipe between them.

To be honest I would hate to think what the cost of the exhaust / cats would be for this - you would probably be looking at a full one at this mileage and unless you are looking at a custom one then the cost alone for the centre piece and back boxes (fitted) by Lexus was £2700 when I enquired 2 years ago when mine went. So, probably add the same again to that. The pattern part is not available from the likes of ATS etc.

Its unlikely both cats have gone ...I think it would be better to have a diagnosis at a dearer unless you would like to replace the sensors for around £200 and see how you get on. But a quick diagnosis would be advisable as you could be damaging the cats which this error code refers to. Also, if you know anyone working at a garage you could ask them to do an emission check for you, if the cats have gone then I would expect it to fail this test.

Let us know how you get on.

My first check would be the exhaust.

ChumpusRex · July 28, 2015

Thanks guys. I've had a brief look and listen at the exhaust and I can't hear it blowing or anything like that.

As I don't want to go taking anything apart at the moment, I did some electronic tests. I manually adjusted the lambda set point and there is a huge discrepancy in the behaviour of the bank 1 and bank 2 sensors.

B1S2 is behaving strangely and seems to be reading leaner than B1S1 reads. This may well be messing up the ECU's testing. I suppose this could be an exhaust leak after the cat, or either sensor could be dodgy.

I might think about replacing the 1st sensors, as they will likely need doing sooner rather than later, and it is a 1st sensor fault that could potentially damage the cat - and see if I can get the exhaust properly inspected at the same time.

ChumpusRex · August 17, 2015

Just thought I'd post an update. My new O2 sensors have arrived from the USA. The code has stayed away for a couple of weeks; came back again, but I cleared it again.

I've also done some more detailed OBD logging and found some old logs from my old laptop.

Anyway, here's a screenshot - as you can see, currently while B2S2 switches slowly, B2S1 switches very fast. I can't think of anything that would cause that apart from a dead cat.

http://imgur.com/RMIeFlj

Even 5 months ago, on my last log, the two traces looked identical. So, I think that it probably is the catalyst, and that something has happened to it over the last few months. Not sure what, maybe it was when there was torrential rain a while ago, and perhaps the cat got quenched when I went through a big puddle.

Thankfully, there are plenty of suppliers of road legal type-approved IS250 catalysts at a good price (I was worried about having to get one from lexus, but the cost of a brand-new aftermarket one with lifetime warranty is a small fraction of what the breakers are charging for a used cat with 80k miles on it).

Will update once I get a proper diagnosis, but now that I've got the sensors in, hopefully, this won't be too much more expensive.

ColinBarber · August 17, 2015

Did you replace any upstream sensors? As I stated previously these can go out of normal operating spec, not enough to throw an error code directly relating to them, buy enough to alter the mixture so that the downstream doesn't behave as the ECU expects.

A faulty cat should be easy to diagnose, just get an emissions test done.

Upstream sensors should switch fast, downstream sensors should switch slow - this is a correct operation. If downstream start switching as fast as the upstream then the cat could be faulty.

Having said all that does the IS250 use an O2 sensor upstream? I thought it used air/fuel ratio wideband sensors - if it does use A/F sensors then your graph is incorrect and your software not set correctly.

ChumpusRex · August 17, 2015

Thanks.

I've got some new upstream sensors in, and plan to change them as they are old, and even if it is a cat problem, it's probably not a bad idea to change the O2 in case that was the cause.

Sorry - typo - the IS250 has wideband upstream sensors. They don't show anything interesting on the graph - the chart I've shown is bank1 and bank2 post-cat sensors. They always used to be identical, but now they are completely different, with bank 1 switching rapidly.

As to emissions, I'll get that done in due course, but the IS250 has secondary catalysts, so even if one of the main catalysts is dead, the emissions may still be passable.

ChumpusRex · August 25, 2015

Interim update:

Emissions tested - passed with flying colours

New oxygen sensors installed

Code and check VSC came straight back.

ChumpusRex · September 1, 2015

Hopefully final update.

New catalyst installed, code staying away for now. A scan of O2 sensor voltages suggests that they are all switching correctly, indicating correct catalyst function.

Got an aftermarket catalyst installed. Cat was £325 and it was about £225 labour from an independent garage. Very, very difficult job complicated by the fact that there were completely seized bolts needing oxy-acetylene treatment holding the 1st cat to the 2nd cat/midpipe. This heat treatment ended up warping and cracking the midpipe. This has been bodged up, but a replacement aftermarket midpipe is about £500. So, hopefully, the repair job will hold.

A lexus OEM catalyst was over £800 just for the 1st catalyst. An OEM midpipe would be over £1000.

No idea what caused the cat to fail. I saw the old part - substrate and everything looked perfect - so probably not a quench (which had been my original suspicion).

normski2 · September 1, 2015

You've had some rotten luck here mate.

Here's hoping all is calm and stable for some considerable time for you.

is200 Newbie · September 1, 2015

That's ridiculous money - I know cats are expensive but 800 for one .... :megaangry:

So for the middle section that would work out to be 2600 - that's a horrendous increase from when I was quoted a replacement from Lexus for mine which was 2700 for the middle and two back boxes.... that was two years ago.

Did the garage have any trouble in removing / re-fitting the downstream sensors?

ChumpusRex · September 1, 2015

That's ridiculous money - I know cats are expensive but 800 for one .... :megaangry:

So for the middle section that would work out to be 2600 - that's a horrendous increase from when I was quoted a replacement from Lexus for mine which was 2700 for the middle and two back boxes.... that was two years ago.

Did the garage have any trouble in removing / re-fitting the downstream sensors?

Yup £800 for a front cat. The midpipe has 2 cats in it which is why it is so expensive.

Well, let's see what happens now. I'll be really disappointed if things start blowing, because that'll mean a new midpipe.

If that is the case, I might have to take it to a fabricator and see if they can do anything with it. I'm guessing that the pipe isn't weldable.

Didn't you get a custom exhaust on yours? What did they do about the cats on the midsection?

ChumpusRex · September 2, 2015

Oh rats.

Exhaust started blowing on the way to work this morning! Took it back to the garage, and they showed where it was blowing from the cracked midpipe.

Their normal supplier was asking £500 for the midpipe, but I've found another supplier (www.skscarparts.co.uk) who can do a road-legal, type approved midpipe-cat with lifetime emissions warranty for about £315 (if I'd known earlier, I could have saved £150 on the front cat). Hopefully, I'll be able to organise an order from them to be delivered to the garage.

As an aside, the same supplier has several different brands of O2 sensors available at surprisingly keen prices. Definitely one to keep in mind, if you need O2 sensors.

As an aside, the rear O2 sensors are easy. The wires are in the footwells. Just pull the middle wall carpet out and the connector, wire and hole in the floor are there.

toffee_pie · September 2, 2015

you can access the rear sensors from under the carpet...?

pics?

ChumpusRex · September 2, 2015

you can access the rear sensors from under the carpet...?

pics?

Not the sensors themselves. The wiring. The sensors are on the midpipes, but the wiring goes up through the floorpan of the car.

On some cars, this can be a nightmare, as seats and carpets have to come out.

On the IS250, the connectors and wiring are in the centre console in the footwells behind the carpet. The wire then heads straight down to a hole in the floorpan. The O2 sensors come with a rubber grommet that fits into the hole.

is200 Newbie · September 2, 2015

I had a stainless steel exhaust made for my car when it went - they managed to keep the old cats to use in it.

I actually bought a second hand centre piece but no one would touch it to put it on - like you have experienced, they feared they would damage other parts when taking the old one off - the bolts to the manifold were a big concern so I sold it on again. (sorry now)

I would strongly research any supplier of these stainless exhaust manufacturers as while the exhaust has been ok on mine and its saved me a hefty wad of cash the downside has been that its noisy. Not so as the boy racer noisy (it has quite a nice rasping sound) but noisy as in a drone between 2-3k revs at around 70mph.

I did do a very good repair on my original exhaust before I replaced it and when it was removed it did look and feel solid enough to last quite a while. Have you got pictures of where its blowing?