Engine Management Light on code P0420

[kuljit55]

Hi guys, i have a 2003 E12 Corolla 1.6vvti Manual. In July 2016 the EML came on and i got it diagnosed. The mechanic gave me the code P0420 which said: Catalyst System Efficiency Below Threshold Bank 1

So he suggested its most likely the Lambda (o2) sensor that needs replacing, it being common on these. So i purchased a Denso Lambda sensor and got it replaced, the car and EML did not come on after, its been over 2 years now and yesterday the EML came on, so i took it and got it diagnosed again, and the same code P0420 has came up! I have got the code cleared for now but would like some advice please. The car was last serviced about 8k miles ago in 2017, and has the original exhaust system.

Has anyone ever had that problem reoccur after replacing the lambda sensor? 

I have read these cars have 2 lambda sensors, could it be possible it can be the other one?

Is it possible for the lambda sensor to become faulty again after 2 years? Would it be wise to change the same one again?

Any advice would be appreciated. Thank you

 

 

[oldcodger]

Hi,

If you web search you will find plenty of info on that  error code though they all seem to concentrate on the cat or using treatments  for it.

Have some error codes with my reader but cannot post the full details as they are overwritten with copyright markers.

However the key info is below.

When you replaced your sensor was it the pre cat or post cat one , and assume being Denso it was a genuine part, the cheapies/clones can give problems as I once found.

What about your MOT emissions readings, are they good ?

You should service every 10k or 12 months whichever comes first.

You can buy simple cheap readers, Android types or standalone  ones for as little as £5 - £20 so you can read off the codes , monitor basic functions and reset the  error lights, well worth it when having such problems.

 

Other causes quoted, in additions to sensors and cat etc   - Leaking exhaust manifold or leak anywhere else on the exhaust system,  Coolant temperature sensor

 

hth

[kuljit55]

Thanks for the reply.

 

When i replaced the Lambda sensor, it was bank 1, so the one at the front end of the car which mechanic accessed from under the bonnet, and it was a Denso branded which i got for Euro car parts. 

As for MOT emission readings, where do i find that information? And how do i determine if the readings are good? I did not get the emissions sheet since about last 3 mot's ago. 

Would it be wise to change the same Lambda sensor again? I dont think i have any leaks in exhaust or cat though its old and the original unit. 

[oldcodger]

Hi,

The emissions sheet should have been given to you at the time of the MOT, if the readings were on the high side , but passable, it might give some clue as to what or where the fault is; but probably too late to get a copy now.

You can just keep putting parts on at £100 at time and hoping, but it would be better to locate the actual fault, though its not an easy one even for a mechanic to pinpoint.

You could replace both front and rear sensors to be sure,  or perhaps have a proper diagnostic test at a Toyota dealer which might show the real fault ??

How many times has the error happened , just once ?  sometimes it can come on once every so often , with no apparent real problem,  just a momentary glitch can cause the error code and light to be logged on.

Whats real problem is when the comes back on straight after its been cleared and affect the engines running.

You could try some of the Cat cleaning adatives like Cataclean etc.

 

 

[DeTomato]

Lambda sensors are VERY easy to destroy. For Example, cleaning it. As long as the holes in the outer shield are ok, then it will be fine, else just use a pin to de-coke it.

If you've put the wrong sensor in, it will flag up this error because of the imbalance between both sensors, front and rear.

[kuljit55]

On 10/29/2018 at 6:13 PM, oldcodger said:

Hi,

The emissions sheet should have been given to you at the time of the MOT, if the readings were on the high side , but passable, it might give some clue as to what or where the fault is; but probably too late to get a copy now.

You can just keep putting parts on at £100 at time and hoping, but it would be better to locate the actual fault, though its not an easy one even for a mechanic to pinpoint.

You could replace both front and rear sensors to be sure,  or perhaps have a proper diagnostic test at a Toyota dealer which might show the real fault ??

How many times has the error happened , just once ?  sometimes it can come on once every so often , with no apparent real problem,  just a momentary glitch can cause the error code and light to be logged on.

Whats real problem is when the comes back on straight after its been cleared and affect the engines running.

You could try some of the Cat cleaning adatives like Cataclean etc.

 

 

Its been 3 days now and the eml light has not come back on. I read that the code for the bank 2 lambda sensor is P0430, so im assuming it cant be the other sensor. I replaced the bank 1 sensor in July 2016 with original Denso one so i wouldnt expect it to in balance with the other one. Should a new lambda sensor generally go faulty after 2 years?

Maybe it was a glitch, im not sure. Will continue to drive and see if it comes on again. Will report updates if anything happens. Thanks

[roks]

Hi,

Give it a few hundred miles, it will come back. I had the same code for ages, I reset it with my scanner tool, you will notice that each time you reset it, the time before it comes back on get shorter, i.e. first time it lasted a year, then it was 6/7 month then 3 months etc...it passed MOT with no issues. In end it was too frequent, so I got fed up replaced both, front and the rear together, its been couple of years since.

Also, my corolla only has one bank, so its labelled as B1S1 and B1S2 on the scanner tool. Watch out for B1S2 (rear), one near the CAT, its baked in, soak it with WD40 or PlusGas or something similar several times over a couple of days. If you don't have blowtorch, drive the car for a while and get it nice and hot, that makes it a bit easier to take off. You will also need a O2 sensor socket.

Don't bother cleaning, it isn't going to work.

Regards,