P0420 - Catalyst System Efficiency Below Threshold Bank 1

[james250]

While driving on the motorway today my check engine light came on. Scanned the code and it reads: P0420 - Catalyst System Efficiency Below Threshold Bank 1. I have new Air/Fuel Oxygen sensors and have had them in the car for nearly a year now. What is this code and any ideas how to fix? For now i've cleared it to see if it returns but as with past experiences with codes, they do always seem to come back until the root cause is addressed. I have an MOT coming up this week so not the best time for such code to pop-up. Any advice?

[scudney]

it could be a blow in your exhaust, near the lamba sensor, had the same when I had a stainless steel exhaust made for my IS200, turned out to be a pin hole in the exhaust near the sensor that kept triggering  the engine management light. Could be a possible leak where the sensor was fitted 

[BachelorDays]

If the light is staying off after resetting it, it's very unlikely it's an O2 sensor or CAT. 

It'll probably come back after a while.  For the MOT, check the day before and reset codes (it might be sitting there as a Pending) - that'll keep it quiet.

Fuel filler cap is the first thing to check - a loose one or one where the O-ring has dried up will cause problems.  If you are used to filling up the tank right up to the brim, it can cause issues with the EVAP system and will likely throw this code.

Beyond this, check the vacuum lines and if you've got a 100k+ on the clock the MAF is always a possibility, but I'd look at the fuel cap first.

[james250]

8 hours ago, scudney said:

it could be a blow in your exhaust, near the lamba sensor, had the same when I had a stainless steel exhaust made for my IS200, turned out to be a pin hole in the exhaust near the sensor that kept triggering  the engine management light. Could be a possible leak where the sensor was fitted 

You mean a hole in the exhaust where the a/f ratio oxygen sensor is? I'm guessing it can also be a pin hole anywhere in the exhaust right? The car has done over 130k on its original exhaust. The pre-cat oxygen sensors are new but the post-cat are still original ones. How do you locate an exhaust leak?

[james250]

1 hour ago, BachelorDays said:

If the light is staying off after resetting it, it's very unlikely it's an O2 sensor or CAT. 

It'll probably come back after a while.  For the MOT, check the day before and reset codes (it might be sitting there as a Pending) - that'll keep it quiet.

Fuel filler cap is the first thing to check - a loose one or one where the O-ring has dried up will cause problems.  If you are used to filling up the tank right up to the brim, it can cause issues with the EVAP system and will likely throw this code.

Beyond this, check the vacuum lines and if you've got a 100k+ on the clock the MAF is always a possibility, but I'd look at the fuel cap first.

It's strange because this happened about 8miles into driving after I filled petrol. I didn't fill the tank though. And I'm sure I tightened the filler cap.

[is200 Newbie]

Take a look at the following web site - it will give you possible causes. See if you can narrow them down

https://www.obd-codes.com/p0420

It does sound like an exhaust issue up towards the sensors and the only way you are going to find out if it is, is to get it up on ramps to see any indications of cracks or rest/soot marks. A lit warning on the dash is an MOT failure.

[james250]

44 minutes ago, is200 Newbie said:

Take a look at the following web site - it will give you possible causes. See if you can narrow them down

https://www.obd-codes.com/p0420

It does sound like an exhaust issue up towards the sensors and the only way you are going to find out if it is, is to get it up on ramps to see any indications of cracks or rest/soot marks. A lit warning on the dash is an MOT failure.

The warning light I cleared it last night. What's the easiest way to see if there is a leak somewhere in the exhaust other than loud noise? I don't the exhaust being loud.

[Texas]

As above to check ( physically) the exhaust you need to get under car and inspect the complete assembly, use a bit of tissue paper and hold it around jonits to show if any gases are flowing out. Soot marks are a tell tell sign if it has been going on for a while.

paul m.

[BachelorDays]

2 hours ago, james250 said:

It's strange because this happened about 8miles into driving after I filled petrol. I didn't fill the tank though. And I'm sure I tightened the filler cap.

Take a close look at it and clean around the filler opening and the cap. Then see how it goes for the next couple of days.  The main thing is to see how quickly the light comes back. If it lasts until the next fuel fill you'll have more confidence in the diagnosis.

[james250]

7 hours ago, BachelorDays said:

If the light is staying off after resetting it, it's very unlikely it's an O2 sensor or CAT. 

It'll probably come back after a while.  For the MOT, check the day before and reset codes (it might be sitting there as a Pending) - that'll keep it quiet.

Fuel filler cap is the first thing to check - a loose one or one where the O-ring has dried up will cause problems.  If you are used to filling up the tank right up to the brim, it can cause issues with the EVAP system and will likely throw this code.

Beyond this, check the vacuum lines and if you've got a 100k+ on the clock the MAF is always a possibility, but I'd look at the fuel cap first.

How long does a pending code stay before it becomes a stored code? If I were to get a pending code show up before the MOT and then I clear it before the car is tested, it will then have to go to pending and stored but while they test the car they leave it on for the duration of the test which is about 45mins and rev it too, so wondering if the light can come back on while they are testing it?

Also, if it was the fuel filler cap, will the error not be more related to something about EVAP? Or is this code also related to EVAP? My fuel cap area is rather dirty, I'll give it a good clean and check the cap. Anything in particular to look for on the cap?

[BachelorDays]

The way it works is if an error is detected it's first registered as Pending. If it persists it becomes active and the light comes on. That's why if it's intermittent it'll stay Pending for a while.

If your light doesn't come on for a couple of days it's not going to cause any MOT problems.

As for the tank filler, dirty means there is some escape of fumes. Just be careful when cleaning lest you drop any residue in the tank. You can examine the cap by feeling the rubber inside. Brittle or dry and wrinkled is normally a sign it's had it's day.

If it turns out to be a fuel cap related problem the clean up will last you quite a while. You can change the cap at a later stage.

Have you driven the car since resetting the light?

[james250]

53 minutes ago, BachelorDays said:

The way it works is if an error is detected it's first registered as Pending. If it persists it becomes active and the light comes on. That's why if it's intermittent it'll stay Pending for a while.

If your light doesn't come on for a couple of days it's not going to cause any MOT problems.

As for the tank filler, dirty means there is some escape of fumes. Just be careful when cleaning lest you drop any residue in the tank. You can examine the cap by feeling the rubber inside. Brittle or dry and wrinkled is normally a sign it's had it's day.

If it turns out to be a fuel cap related problem the clean up will last you quite a while. You can change the cap at a later stage.

Have you driven the car since resetting the light?

I reset the code last night one the way home and drove home which was about a 15min drive. Will clean fuel cap area while the cap is closed and then inspect it so dirt doesn't fall in and go for a drive tonight. After that if light hasn't come on I'll plug in the reader and see if it's sitting as pending 

[BachelorDays]

Sounds good. I'd clean around the filler opening as well as any deposits here will inhibit the cap from sitting air tight.

You could always check for the code before your next run.

[is200 Newbie]

For some reason I thought the fuel cap fitting issue was specific to the diesel where the system would lose pressure in the tank - maybe wrong - wait to see if any diesel owners chip in...

[The-Acre]

When my GS O2 sensor started playing up I'd erase the code but it would come back within a couple of days, sometimes within minutes.  A pinhole in the sensor area wouldn't be noticeable audibly, unless it's a very big pin!

[BachelorDays]

2 minutes ago, is200 Newbie said:

For some reason I thought the fuel cap fitting issue was specific to the diesel where the system would lose pressure in the tank - maybe wrong - wait to see if any diesel owners chip in...

Not specific to diesel. Had one for ages on my Mercedes E Class. Had it come up on my RX 350 too.

[The-Acre]

3 minutes ago, is200 Newbie said:

For some reason I thought the fuel cap fitting issue was specific to the diesel where the system would lose pressure in the tank - maybe wrong - wait to see if any diesel owners chip in...

My diesel Volvo has a warning on the cap saying that it must be turned to click to avoid system problems.

[is200 Newbie]

20 hours ago, james250 said:

While driving on the motorway today my check engine light came on. Scanned the code and it reads: P0420 - Catalyst System Efficiency Below Threshold Bank 1. I have new Air/Fuel Oxygen sensors and have had them in the car for nearly a year now. What is this code and any ideas how to fix? For now i've cleared it to see if it returns but as with past experiences with codes, they do always seem to come back until the root cause is addressed. I have an MOT coming up this week so not the best time for such code to pop-up. Any advice?

In your post you have stated that you have replaced the sensors ... Have you replaced all 4 or just 2 of them.. The Air/Fuel sensors are in the engine bay, the air sensors are on the exhaust. Highly unlikely the exhaust sensors have been damaged from the install of the new ones.

My experience of a failing sensor is that it will initially go pending (will light up and can be removed) after a few days it will come back again and may be removed but after that it will come on and stay on, makes sense if its on the way out as it probably wont fail immediately. If you can, I would get it to the likes of ATS or somewhere and ask them to do an exhaust check - should only cost a few quid. If this is the original exhaust then it is ripe due to the age and mileage of the car. I would still get the fuel cap seal checked out as mentioned above as that's easy but for piece of mind do the exhaust as well.

If the readings in comparison from both sensors, the air/fuel vs the air sensor are the same or the air sensor reading is higher then theres an issue with the cat or the exhaust between the sensors as the air sensor should have a lower reading than that of the air/fuel as the cat should be working right. The air/fuel sensor will adjust any anomalies with the reading from the air sensor until it reaches preset band of errors - if it cant then theres an issue.

I cant find one at the moment but if you look at the mountings of the exhaust - I don't think they are sufficient - should the exhaust fail and drop at that point I would hate to see the damage that would cause.

[is200 Newbie]

I have circled the air sensors and the approx. location of the air/fuel sensors in the engine bay - would give you a pointer in where to check. see image below ....

[james250]

1 hour ago, is200 Newbie said:

In your post you have stated that you have replaced the sensors ... Have you replaced all 4 or just 2 of them.. The Air/Fuel sensors are in the engine bay, the air sensors are on the exhaust. Highly unlikely the exhaust sensors have been damaged from the install of the new ones.

I replaced the 2 Air/Fuel ratio sensors in the engine bay. Fuel economy has improved since. Haven't touched the sensors post-cat though. 

[james250]

Been out for a drive. As it was raining and dark I didnt want to risk getting water and dirt into the fuel tank. so i just cleaned the area and checked to make sure cap is tightened. I had a quick look at the exhaust from underneath at the back and couldnt see any soot or marks but need to check the entire exhaust in the daytime. Before I started the car, I scanned it to read the pending codes since I cleared it before driving home yesterday. There was 0 pending codes, so I started up the car warmed it for few minutes and then drove for about half an hour until engine reached its temperature. Came back home turned the car off and plugged in the scanner and it still reads 0 pending codes. Not sure how long it will stay like this I hope it doesn't throw up the error at the MOT test. 

[james250]

On 12/23/2018 at 10:44 AM, BachelorDays said:

Take a close look at it and clean around the filler opening and the cap. Then see how it goes for the next couple of days.  The main thing is to see how quickly the light comes back. If it lasts until the next fuel fill you'll have more confidence in the diagnosis.

So I just had the MOT and it passed. They didn't flag any issues. The light has been off since i first reset it in Saturday. I've been checking to see if it's pending but nothing is pending on the system still. I checked it just before giving car in for mot and there was nothing pending still. Will keep an eye on it and see if it comes back as pending. I'm thinking to get the entire exhaust checked at Kwik fit they seem to offer a free exhaust inspection and report. The exhaust just looks really rusty but other than that I don't see any spot or any marks. I don't know if it's just me but I felt like I needed to rev harder to get the rev up as apposed to a gentle tap on the throttle 

[Texas]

Kwik-Fit... Hhmmmmm... I guarantee that they will find something 'wrong' with your exhaust but then again they won't have one on the shelf to sell you so let us know what they do actually say.

re-wind...you just said it passed an MOT ( with no EML on!) so exhaust is in a serviceable condition.

Note: the factory fit exhaust on the 250 was heavy gauge mild steel and not stainless steel, these OEM exhausts can last a good number of years even when they actually look rusty, If you get a stainless system fitted at some point then be warned the sound is not going to be the same!

paul m.

[james250]

16 hours ago, Texas said:

Kwik-Fit... Hhmmmmm... I guarantee that they will find something 'wrong' with your exhaust but then again they won't have one on the shelf to sell you so let us know what they do actually say.

re-wind...you just said it passed an MOT ( with no EML on!) so exhaust is in a serviceable condition.

Note: the factory fit exhaust on the 250 was heavy gauge mild steel and not stainless steel, these OEM exhausts can last a good number of years even when they actually look rusty, If you get a stainless system fitted at some point then be warned the sound is not going to be the same!

paul m.

I do not trust Kwik fit either to be honest but seeing as they offer free inspection, I was hoping if there is any holes they can identify it for me. Otherwise I guess I can go under the car and have a good loook. In terms of the rusty exhaust is there no way to clean it up and maybe paint it with high temperature black paint?

The EML hasnt come back yet, I will keep an eye on it. One thing I've noticed recently is that when starting up in the cold after car has been sitting overnight. It starts at a certain rev and then the rev drops and then surges back up. So almost creating a slight shiver. This happens and then the rev becomes stable this all happens for the first under 1min and then it becomes normal. Will a leak in the exhaust cause this?

[james250]

So the error after a while now is showing as pending. Two things I've noticed, firstly when starting the car the exhuast does have a ever so slight louder growling sound. It sounds as if it's coming from under the front seating area. So probably a hole in the exhaust somewhere there? Also after running the car when I stand near that area it smells like fuel, does an exhaust leak create petrol smells?