Cat and 02 sensor conundrum

[Yorkshire GOC]

As seen in previous postings in December/January i had a bad misfire issue. Was resolved when coil packs changed and spark plugs. However in May first time ever she failed the MOT on emissions. So working on the assumption that the misfires (which had got bad and quite often) had damaged the cat (ignition misfires do the most damage to a Cat) i swopped out the Cat.

Result she passed the MOT. However a couple of weeks ago i got the error code PO420 - "Cat below efficiency threshold" pending. Tried a few days later with the code reader and the code had cleared itself.

So i used my code reader to see the pre and post cat O2 sensor voltage signals on a 40 mile journey - pre cat O2 sensor was spot on - the post cat sensor (which should run rich and have gentle spiking at most)looked like the pre cat O2 sensor with massive spiking from rich to lean -which means in my mind the Cat is not working to clean the exhaust gases.

Luckily covered by warranty - mmm -so i thought duff after market cat or something not right with the engine ie misfires. So i then ran on a 20 mile journey with the code reader set to the short and long term fuel trims -for an engine of its age the readings were perfect - varying from plus 3 to minus 3 with plenty of zero readings. Also the car is running OK with rock steady idle - however MPG is down noticeably.

So back to either a duff cat or a duff post cat o2 sensor. Did another 20 miles today and for most of the journey the post cat o2 sensor reading looked OK - with a few extreme lean readings 2,500 -3,000 rpm but not all the time.

So would be interested what folk think - is it a duff after market catalytic converter or is something wrong with the post cat 02 sensor itself.

[Ducati750cc]

So i used my code reader to see the pre and post cat O2 sensor voltage signals on a 40 mile journey - pre cat O2 sensor was spot on - the post cat sensor (which should run rich and have gentle spiking at most)looked like the pre cat O2 sensor with massive spiking from rich to lean -which means in my mind the Cat is not working to clean the exhaust gases.



The pre cat sensor should oscillate between rich and lean, the post cat sensor ( downstream) however should literally flat line once the cat has lit off.



The MPG being noticeably down suggests, obviously a rich mixture, and the spikes you see at the post cat could be caused by the mixture being so rich that the cat can't cope with the excess, the post cat sees this and signals it back to the ECU.


The thing is, did whatever is showing up now cause your original cat to fail after driving for months after sorting the coils out ?


The original misfiring could have damaged the pre cat sensor.


The code you got is generated by the signal from the post cat sensor to the ECU which appears to be doing its job, the pre cat sensor, the one the ECU relies on when in closed loop may have been damaged and just about operating within its parameters possibly sluggishly which won't trigger an error.


There are other reasons for a excess fuel consumption that will need to be ruled out.


Were both sensors changed with the cat?


Ideally a scoped reading of both sensors voltages high to low and speed of transition would give clues.

[Yorkshire GOC]

Originally Posted by Ducati750cc

So i used my code reader to see the pre and post cat O2 sensor voltage signals on a 40 mile journey - pre cat O2 sensor was spot on - the post cat sensor (which should run rich and have gentle spiking at most)looked like the pre cat O2 sensor with massive spiking from rich to lean -which means in my mind the Cat is not working to clean the exhaust gases.



The pre cat sensor should oscillate between rich and lean, the post cat sensor ( downstream) however should literally flat line once the cat has lit off.
The precat oscillates as it should from rich to lean - and the pre cat O2 sensor was replaced when the new cat was put in.



The MPG being noticeably down suggests, obviously a rich mixture, and the spikes you see at the post cat could be caused by the mixture being so rich that the cat can't cope with the excess, the post cat sees this and signals it back to the ECU.
[COLOR="Red"]Absolutely but the short and long fuel trims are spot on in fact near perfect - so no excessive rich output mixture from the engine

[/COLOR]
The thing is, did whatever is showing up now cause your original cat to fail after driving for months after sorting the coils out ? [COLOR="red"] I doubt that - the misfires were heavy and over 3-4 weeks increasing in frequency and and ignition misfires do most damage to a cat as they overload O2 into the converter -so am pretty confident the original cat was damaged by ignition misfires.


The original misfiring could have damaged the pre cat sensor.
Pre cat sensor was changed along with the Cat.

[/COLOR]
The code you got is generated by the signal from the post cat sensor to the ECU which appears to be doing its job, the pre cat sensor, the one the ECU relies on when in closed loop may have been damaged and just about operating within its parameters possibly sluggishly which won't trigger an error. Mmm - yes so if thati s the case it is looking like the Cat itself in my mind - on my first test the post cat sensor was way out of kilter - oscillating wildly - when it should vary only slightly and run rich - it was all over the place and looked like the pre-cat sensor and am sure would have failed emissions test . On the 2nd test the post cat sensor performed in more normal parameters but still fluctuated to lean at 2.500-3000 rpm. This , asuming the post 02 sensor is OK means the cat is throwing out unclean fumes


There are other reasons for a excess fuel consumption that will need to be ruled out. Agreed but it is a classic symptom of a failing Cat / or 02 sensor. So i think when i get a 420 code and a perfect short/long term fuel trim, normal pre 02 sensor reading its either the cat or the post cat O2 sensor.


Were both sensors changed with the cat?
No just the pre cat sensor - but as the Cat is under warranty i might just replace it and also the post cat sensor - will check the post cat O2 sensor wiring/fitment

Ideally a scoped reading of both sensors voltages high to low and speed of transition would give clues.

Agreed - but don't think my reader can show this - but certainly on the first road test and partially on the 2nd road test the post cat 02 sensor is pointing to a duff cat or something wrong with the sensor itself.

[SD1too]

Hi Martin,

This is interesting because I had a very similar experience, not with my Rover 75 but with my other car. The ignition module failed causing a misfire. It was renewed and I subsequently got the catalytic converter low efficiency fault code. I fitted an aftermarket replacement (the Citroën part was an absurd price) with a new post-cat sensor and it was fine for about a month then the code reappeared. My fuel consumption is fine and it's never failed the MOT on emissions (although the results are not ideal) but the 'check engine' light won't stay extinguished for longer than a couple of days. It's been like this for about 5 years!

I'm sorry that I don't have an answer for you. I do wonder though, whether these aftermarket catalytic converters contain less of the active precious metal ingredient to keep the cost down.

Simon

[Yorkshire GOC]

That's an interesting thought Simon- thanks - i have 10 months on the warranty so will investigate further - there is a very local cat testing garage a mile away - i may ask them to see if the Cat is OK or not.

[dave lincs]

Hi Martin hope you are well, p0420 is the catalyst not working well enough quite normal with cheap aftermarket ones they are not up to standard that your car needs to stop light coming on

If you still have the original then might be worth putting it back on and giving it a good run

[Ducati750cc]

Agreed - but don't think my reader can show this - but certainly on the first road test and partially on the 2nd road test the post cat 02 sensor is pointing to a duff cat or something wrong with the sensor itself.


Most household code readers don't have a scope function, a good fuel injection specialist would be able to watch and graph the voltage outputs from both sensors, how they cycle and how quick they react and the waveforms, comparing them with known good waveforms stored in the equipment.



Once up to temperature the pre cat sensor should vary between 0.1v and 0.9v ( Zirconia ) or 0.4v to 4.0v ( Titania ).



The post cat should have have a steady output of around 0.1v ( lean mixture out of the cat not out of the engine ) with possibly the occasional rise if the cat output goes rich.


A pre cat sensor provides info to the ECU as to the make up of the exhaust gasses which the ecu adjusts accordingly.


A post cat sensor is a diagnostic one monitoring the effectiveness of the cat.


on my first test the post cat sensor was way out of kilter - oscillating wildly - when it should vary only slightly and run rich - it was all over the place and looked like the pre-cat sensor and am sure would have failed emissions test .No, the post cat, if everything is working as it should, should be constantly lean, with just the occasional blip to the rich side, if it shows constantly rich, then the cat, for whatever reason, isn't cleaning up the gasses. On the 2nd test the post cat sensor performed in more normal parameters but still fluctuated to lean at 2.500-3000 rpm.As it should do ! This , asuming ( never assume always test and verify )the post 02 sensor is OK means the cat is throwing out unclean fumes Or.... the post cat isn't OK, the ECU is working on incorrect values from the sensor and consequently producing a rich mixture that the cat can't cope with.


and ignition misfires do most damage to a cat as they overload O2 into the converter No, the thing that damages the cat is not oxygen but unburned fuel which both pollutes and raises the matrix design temperature.

[Yorkshire GOC]

Originally Posted by dave lincs

Hi Martin hope you are well, p0420 is the catalyst not working well enough quite normal with cheap aftermarket ones they are not up to standard that your car needs to stop light coming on

If you still have the original then might be worth putting it back on and giving it a good run

Hello Dave,

Hope you and the badgers are well -was in Cheltenham yesterday with friends who also have a badger set in their garden. It did occur to me Dave loads of US sites advise don't buy after market Cats. Afraid am stuck with this one - will get it tested i think.

[Yorkshire GOC]

I always understood there should be less free O2 leaving the converter than entering it - giving a Rich bias to the post cat O2 sensor. ?

A straight cut and paste using the search term "post cat o2 sensor voltage"

post-cat oxygen sensors, when good, feature a steady voltage usually between 500 to 700 mV. If it zigzags, the catalytic converter is highly suspect.

The Site below shows a live post cat test showing a normal reading - and the post cat should give a pretty constant Mv figure and oscillate little - Mr Google say 0.5 to .08 - as you see in this example. And he is demonstrating a working post cat o2 sensor - the pre cat O2 sensor oscillating constantly which mine is doing but also my post cat sensor is zigzagging as per the above cut and paste from Mr Goole.- according to the guy on you tube if that happens your Cat is suspect assuming the post cat sensor is OK.

But i take onboard Dave Lincs/Simons comments comments about the efficacy of after market Cats and this may explain the code

https://www.youtube.com/watch?v=rWIpwNI3cAU

[dave lincs]

Originally Posted by Yorkshire GOC

Hello Dave,

Hope you and the badgers are well -was in Cheltenham yesterday with friends who also have a badger set in their garden. It did occur to me Dave loads of US sites advise don't buy after market Cats. Afraid am stuck with this one - will get it tested i think.

Yes all good here thanks Martin we have 2 baby badgers with us every night for a good feed up (Dont tell Trikey or he will be round after dark for a snuffel)

Your car definitely needs the correct euro standard catalyst as although it will work and pass the mot it is not good enough to stop the mil light coming on showing p0420 early cars did not have a downstream sensor so it not be a problem on an earlier car, get in touch with wo supplied the catalyst and ttey should change it for the correct one

Dave
Hope we will see you at the east kirkby meet