Error Code: P0420 - Causes & How To Fix?

[mickburkesnr]

While driving my 2010 1.0 107 (same as the Aygo) back from Cardiff a few weeks ago the engine light came on. I've had the car since new, it's done 63,000 miles and regularly serviced by myself, and it's never had a warning light come on in its life. I got it home, plugged in my bluetooth OBD2 reader and it gave me the error code P0420. I cleared it - didn't reset the ECU though - and it stayed off for about a week. Then it came back on. On Friday gone I reset the error code again and drove from Stourbridge to a wedding in Brize Norton, and then from there to Northampton on Saturday morning. On Saturday afternoon I drove a 30 mile round trip from the hotel to another wedding with a mix of A roads and motorways, and the light came back on. Same procedure as before, I reset the code but couldn't reset the ECU due to lack of tools.

I have now had chance to read up on the code, and from what I read it could be down to either the catalytic convertor or the post lamba sensor. At the moment it's difficult for me to do any sort of manual work with the cars as I've torn something in my knee, so any DIY or project I have to do in this time of waiting for MRI scans and operations I need to plan out properly as I don't want to spend forever doing one job only to repeat myself. I know a sensor is fairly cheap and easy to replace, but the cat isn't as easy or as cheap. So I was wondering if there was any way I could test to see if the catalytic convertor itself is collapsed or at fault without having to get under the car, change the sensor, then wait to see if the light comes back on?

For the record the catalytic convertor was changed over a year ago with one from Euro Car Parts. I'm not sure if anyone here as ever seen them, but you buy them and they look like some guy has welded it together in their garage. It has been a bit down on power since then, but before the cat was replaced it was parked for 18 months while I drove a far more powerful car, so I've never been able to work out whether its the car thats down on power or me being unable to tell the difference.

[Planemo]

Hi Mick,

Theres no easy way around this I'm afraid. And even if/when you get the cat off, due to the integrated design with the manifold theres not a lot of visual inspection you can do. You can't see anything of the top of the cat and even peering into the bottom where it joins the downpipe doesn't show you that much. You could of course try using boroscopes or similar but unless you can borrow one its added expense. That said, if you get it off and it rattles like crazy you have found your problem.

Even though your pattern cat is relatively new, I am not a fan of non-OE cats and it may well be that it has collapsed or indeed simply become inefficient due to low quality materials used within. If it were me, I would have gone for a used OE cat off a low mileage car rather than a new pattern one as theres loads available on the Bay and they are not a lot of money.

The other way of attacking this is to simply to fit an o2 extender as I have done in my decatted Aygo. It will prevent the ECU light coming on but I would not advise it on the basis that we don't know if your cat has collapsed which could present a serious restriction. Although it would be unlikley to cause engine damage, the loss of power could be considerable and with the Aygo not exactly flush with power I prefer to get every HP I can!

Alternatively, it could be the sensor as you suggest but they are fickle things to test accurately. Again, I do not like pattern sensors despite their attractive price. I have yet to see any that have lifespans anything like OE and most have failed on me within 2 or 3 years. I would rather buy OE and forget about it for a long time.

I wish I could give you an easy answer. I think the way I would approach this (assuming you want to keep the car legal and run a cat) is to replace the sensor with an OE one and see what happens. If you still get an error code ditch the pattern cat and get a good used one off eBay. Changing it is a lot easier than some cars but I do appreciate your mobility issues. Remember to have a new manifold and downpipe gasket to hand too. Good luck!

[mickburkesnr]

I had feeling that would be the case.

I've had a look at the price of the OEM sensors, it's the cheaper option and at least if it isn't the fix to my issue at least it'll be one less thing to sort out. I'm going to try and give the cat a bang with my fist to see if there's a rattle, the only other thing I can think of is if the car smells rich without the light being on, might be a sign of a failed cat?

In terms of de-catting, I'm going to have to keep it fairly legal, but apart from the improvement on MPG is there any noticeable difference to engine response? I'm curious about whether it gives any performance improvement.

[Planemo]

I agree Mick, your sensor is 6 years old so no harm in changing it anyway if the car is a keeper.

As for banging the cat, it might show something but with an engine hanging off it I am not sure you will get the required movement but worth a go.

In terms of richness, you are on the right track. It wont necessarily be richer (as mine is definately using less fuel given the same driving) after the decat but the smell from my pipe (ooh err) is absolutely different to a catted car. Think pre-cat days (if you are old enough) and you would be bang on the money. Mine smells acrid to me now, far more offensive and reminds me of my days in the workshop with pre-cat cars flat out on the dyno. I hadnt thought of this potential diagnosis and you could be onto something. Worth a sniff for zero cost I guess.

As regards to decatting, there is definately an improvement with power in the mid range but my head is struggling to understand why this is, given the top end seems no different. I wish I had a before and after dyno run to confirm but my seat of the pants dyno tells me that there is more torque in the midrange. I cant put my finger on it especially as any removal of restriction would normally only show up in the top end but its certainly there from about 2k rpm to 4k rpm. Oddly, I cant say that top end has changed at all. I dont know why I have the feeling I have, given that the fuel and ignition maps wont be any different when the ECU has reached open loop. Maybe Toyota mapped the car to be richer in that sector and removing the cat lets it breathe better but this does not explain why the top end feels no different to me, unless the OE map is lean after 4k. I would love to map this little car (inc the VVTi) to see what we could get but it would be a £1000 expense that I cannot justify. I believe that richening after 4k rpm with a decat could show good gains.

If there is any way you can 'manage' the MOT side of things I would definately reccomend you consider a decat. You could always decommission your current cat and submit it to the 'Gods of Hammer' seeing as you may be purchasing another cat anyway...

Nathan.

 

[mickburkesnr]

I vaugely remember smelling cars that didn't have cats, but I do remember the old fella having a Rover 820 and the catalytic convertor was rattling around and it smelt funny. The previous owner was one of those owners who decided to put 4 Star petrol in to their cars instead of unleaded during the petrol strike years ago. It destroyed the cat and he had it replaced.

I think with the exhaust system on cars if the gases aren't being removed properly then the engine will struggle to burn more fuel and air to then push on through the exhaust if that makes sense? If theres a block or if the gases take longer to pass over the catalytic convertor then it's going to hold up everything else behind it. That's why I'm starting to think the lack of power is down to this cat not being very good.

I'm going to buy the sensor anyway next week and see what difference it makes. The engine light hasn't come on since I reset it and it's done 100 miles so far. It's definately a keeper, first car I ever had new and the thing has brought me as far east as Oban in Scotland and as far west as Budapest. I'd be a numpty to get rid of the thing!

[mickburkesnr]

So I tried to do the smell test, I couldn't smell anything that indicated any issues with the emissions. I put my hand on the exhaust and I could feel the gasses being "popped" out while idling. Not a stream of gasses but definate pulses of gas being popped out. Not entirely sure if that's to be expected from a 3 pot engine?

[Planemo]

The Aygo does tend to pulse and its due to it being a 3 cylinder as you say. Its even more noticeable with a free flow rear box, I can actually hear each pulse on idle. May be that you just need a new o2 sensor then. Fingers crossed.

[mickburkesnr]

Hopefully. I get paid next week so I'll order a new one then and update every one.

Should point out that it's now Thursday, 250 miles more since I reset the light and it hasn't come back on. So it's making me think it's a sensor issue.

[eygo]

i used to know a guy who worked in one of the high street tyre / exhaust centres and he told me they used to

put a bar into the cat and hammer it to break the cat up and then remove it through the end pipes

this way there is no signs its been tampered with,just an idea for de catting

[Planemo]

It would be incredibly difficult to do this with the Aygo maniverter due to the fact that you can only work from one end and even then it is a very small hole to work with!

[mickburkesnr]

Thing is though with the catalytic convertor I bought it looks like it's been made up in a garage. Like the pipe is clearly welded to the body, the body is clearly welded together. It looks every inch a £120 part. So in this respect I could take it off, cut it at the back (somewhere hard to view), gut it, re-weld it and put it back. So yeah it's very much doable to gut the cat, leave it there (maybe get a pipe to run straight through from the top to the bottom of the cat so there's no blow back?) and shrug my shoulders when the guy says it's over the CO2 level.

I'm assuming that would be an issue after though? Would the increase in the CO2 make it an MOT fail?

[Planemo]

Its an MOT fail if the co2 is over, whether you shrug your shoulders or not. That said, I have personally seen some decatted cars scrape through believe it or not, but this was several years ago and I do not know if the limits have changed since then. I have yet to find out whether the Aygo is one of the cars which could still scrape it.

[mickburkesnr]

I've been thinking about this since I posted it. The light's come back on, and the last two days I've done short journeys and decided to listen to the engine properly. When accelerating there's a metallic buzzing sound, and when I go off the accelerator it stops. It sounds just like it did when the catalytic converter broke in the first place, just not as loud. The weld where the down pipe from the manifold meets the cat had broke. So I think now, really, the cat is at fault and knackered. Fuel economy has gone through the floor really, the performance is like an old diesel without the turbo. I'm talking about one of those old Bedford lorry diesels that the army had in the 60's.

The MOT isn't due for another year, so I'm wondering whether to gut the catalytic converter, stick a pipe going from the entry pipe to the exit inside the cat (so the air flow isn't disrupted - maybe), and work out how to get rid of the engine management light. It's a 1.0 litre engine, the CO2 can't be that bad without the cat surely?

[Planemo]

It does sound like your cat is struggling. For what a good condition used one costs, I would bin it anyway.

Personally I wouldn't entertain trying to fit a straight pipe through the centre of the stock maniverter. The overall design just doesn't lend itself to it being a feasible option. If I was that bothered about it I would buy a proper tubular 3-2-1 manifold and be done with it. I did try and find one before I butchered mine but couldn't find one anywhere and a custom made option would just be too expensive for the power returns. As it stands, the stock one with just the cat removed does well enough for me.