Completely confused by P0420 on my turbo '2.....

[andywood]

Hi All

Any help on this would be really appreciated as i am now losing the will to live with this problem and pulling out way too much hair!!

Bit of a story and summary of what has been investigated so far, apologies in advance if too much info to keep you entertained.....
Have been searching and researching lots on this, so hopefully all the below will make sense and show some vague structure to my fault finding to date!

The problem
Recently my car has started to occasionally flag a MIL for the first time in its life (must be rebelling at my thoughts of selling it!). Read the code and was reasonably relieved to see that it was a P0420 (Catalyst efficiency) which i just put down to the sensitivity of that diagnostic to running a sports CAT. However i was keen to investigate further to ensure that everything was as it should be on the car.

Usual causes of this problem
This code is normally flagged when the post CAT o2 sensor sees a similar situation to the manifold o2 sensors and assumes therefore that the CAT ain't doing what it should be. Manifold sensors should constantly oscillate rich/lean, post CAT sensor should be relatively steady output. If the manifold and post CAT sensors have similar outputs then the diagnostic is flagged (with other entry conditions satisfied such as coolant temp etc....) as assumes that CAT not doing what is should be.
Majority of the time a 'real fault' causing this is down to a knackered CAT, leak in exhaust or knackered o2 sensor. So the investigations began.......

Car setup for info
Full SP turbo (high output) with sports-CAT and mapped by Redline tuning.
Now done approx 6k miles since turbo and sports CAT fitted (new condition as fitted).
No MIL ever or since turbo fitted and passed MOT emissions no probs back in Dec-10.

When MIL/P0420 seen
Is very intermittent it seems to only come on during or just after a long steady run (i.e motorway). No other noises, problems or strange happenings. Engine runs perfectly and no other fault codes flagged.

Investigations so far
OBD tool connected up during idle and higher rpm shows that all the o2 sensors are functioning. Also confirmed that everything else on car seems to be ok, MAF signal, coolant temp signal, short term and long term fuel trims ok etc.... Basically the engines seems to behave and look like it is running perfectly in control!
No flags have been seen for misfires or any to suggest that it is unable to control fuelling or any other code relating to o2 sensors.
Found a minor leak in turbo/manifold gasket and this was replaced and no leak now confirmed (I think this was v minor and just distracted me from the real cause it seems).
Entire exhaust system from manifold back to silencer has been checked for leaks with no concern.
No oil consumption or coolant use, no inlet leaks suspected as would be noticeable when on boost.
Assumed that no way that catalyst could be cause as is only 6k miles old, thought best to take to local garage for emissions check to make sure that all seemed ok.......

This is where it really starts to get strange.
At idle (exhaust pipe measurements) - AFR varies from 20-25!!, CO (0.4%-0.5%) and HC (400ppm) are relatively high (compared to last MOT result at same garage), therefore AFR says lean and the rest says rich!
At high idle (3000rpm) - AFR sits nicely at approx 14.7, CO and HC are still relatively high but do come down to better position compared to idle (I put most of this down to CAT temp increase and becoming more efficient).

Got an infrared heat gun on the CAT and showed after some revving that inlet side temp as approx 225degC and outlet side temp was approx 190degC. My understanding is that CAT should show a temp increase across it, as I have a temp drop it suggests a crap CAT, but the CAT is only 6k miles old and the post CAT o2 sensor does give a steady reading most of the time.

Further checks on the OBD tool showed that manifold o2 sensors are continuing to oscillate from rich to lean as they should, but the post CAT sensor seemed to do the following:
At idle - steady 0.5-0.7V (as normal) but then frequent dip down to 0.0V!
At fast idle (3000rpm) - steady 0.5-0.7V but occasional dip down to 0.0V!

Conclusions so far, if you could call them that..!
Engine seems to be running very well, thinks it is running well, sensor outputs say it is running well, but tail pipe emissions are saying something else at idle and a little bit at higher rpm.
CAT having a temp drop across it at running temp is very suspicious of a knackered CAT but this would not explain the very strange AFR at idle and I would expect the p0420 to flag more regularly if that was the case.
O2 sensors all seem to be doing their job correctly but the post CAT sensor occasionally is dropping to 0.0V (more frequently when at idle) which i am currently struggling to understand/explain.

If the CAT is knackered then I can just get it replaced, but the key here I think is to understand why it may have become knackered so quicky and why the engine is showing such strange happenings at the tailpipe. Or I have completely missed or overlooked something else.
Every scenario I have thought of though would result in an obvious problem, noise or behaviour on engine, or would be noticeable by p-code flag or interrogation by OBD tool.

I think that for a next step I am going to have to make the trek over to Richard at Redline in Essex to see what he thinks may be going on, however any suggestions of other things to check (incase it is an obvious or easy fix) would be much appreciated.

Cheers

Andy

[Wabbitkilla]

Obviously you're running a piggy back to the ecu here ... have you reset the ecu and see if it stabilises for a while?
Sounds like some contention is going on

[andywood]

Reset done a couple of times now, but as i understand it the piggy-back is only really controlling things (fuelling) on boost so the rest of the time it is down the stock ecu so shouldn't be seeing any conflicts.
Idle and steady state running (closed loop) should be down to all the usual stock engine gear and ecu.

Andy.

[andywood]

Just remembered one thing that i forgot mention in original post....

Not sure how normal this is for a turbo'd car with sports CAT, but when the car is hot and on a good run i do get a bit of popping/back-firing from the car when on lift-off/overrun. Not to the extent that there are flames licking out the exhaust pipes, but just a little pop you can hear going on in the manifold. You can even get it to repeat sometimes if you time a throttle blip and lift-off at the right time.

I know that backfires can damage CATs, but I assumed that this was only the case when they were regular and occuring even on normal gentle driving lift-off. This is really not the case on my car and you do have to really try to get one.

Maybe a new CAT and remap is the only choice to be sure that all is defo fine with everything that is happening after the engine (as am pretty confident that the engine itself is running just fine).

Andy.

[aaronjb]

If I were you I'd try swapping out the O2 sensor before the CAT - my 'other' car does exactly the same thing and pegs to either ~0.0V or 0.7V depending on how it's feeling, then goes back to cycling normally for a bit. I know the CAT is just fine (passes the MOT with flying colours), so I'm guessing the sensor is dying..

Bound to be cheaper than a CAT, too..

[andywood]

Cheers Aaron, but the sensor which seems to randomly drop to 0.0v every now and then is the post CAT one. The manifold ones seem to be working just fine. Shouldn't see any real oscillation in the post CAT one anyway.

Thermal scan on the CAT also suggests that it has degraded so i think that the post CAT sensor and the p0420 diagnostic is probably doing its job just fine and flagging that the CAT performance has dropped right off.

Sorry if i have misunderstood your point though, lots of different scenarios flying round my head at the mo!!

Andy.

[aaronjb]

It's my understanding that the post cat sensor should still cycle between lean & rich, just more slowly than the pre-CAT sensor; at least that's what I see on the pair of sensors that are working OK on the Omega (V6 - two pre-CAT O2s and two post-CAT O2s, one bank behaves fine, the other throws random P04xx errors).. while the side that throws errors behaves as yours; randomly pegs to a voltage then suddenly starts cycling again.

More noticeable when racing the engine at 2-3000rpm, though.

[Mike_V]

Andy.
I threw the same code when I replaced my full exhaust system and like you it only showed on long journeys, I was never sure what it was and never really found out. My new exhaust system and CAT had only covered 600 miles so I would rule out the Cat, I then had my turbo, new exhaust, ECU etc fitted and it no longer showed. On inspection of my original downpipe which was fabricated by a local garage the MIG welding was crap to say the least so I put the code down to leaks in the downpipe or post cat O2 sensor. The spitting and popping was a normal feature of the back box of which I had no problem with, maybe no help to you but that was my experience with a P420.

Mike

[andywood]

Yeah you're right, for the sake of 60notes or so is probably worth definitlely ruling out the o2 sensor for good and normally a swap-out gives a pretty clear answer. Still convinced that something is not right or has degraded with the CAT though based on other stuff I am seeing. Best to change one thing at a time i suppose and see what shows next.
Will have a look where I can pick one up quickly and cheaply.....

Cheers

Andy.

[Mike_V]

Always start with the easiest option and work your way through, that's my advice anyway.

Mike

[Anonymous]

Ive got a post cat sensor, £25 incl pnp. I know a new one would be ideal, but they aint cheap.

[Anonymous]

sparkplugs

[mrzwei]

sparkplugs

Very well spotted. If there is a reference then my apologies, but you really do need to get the simple stuff out of the equation first.
I'm not into this these days but increased engine temps etc because of modifications can call for a different heat range of plug.
I'm sure your tuner accounted for this but rbuckingham is right to flag it up.

[andywood]

sparkplugs

Very well spotted. If there is a reference then my apologies, but you really do need to get the simple stuff out of the equation first.
I'm not into this these days but increased engine temps etc because of modifications can call for a different heat range of plug.
I'm sure your tuner accounted for this but rbuckingham is right to flag it up.

Guessing that he might have been referring to the online store selling cheap o2 sensors rather than the little sparky things in my engine that do the magic. Forgive me if I am wrong though and i will be checking them to see if they show anything, but the right grade were installed with the turbo.

Andy

[Anonymous]

ment the store but now its mentioned poss plugs but then the first o2's would see it as rich. now i dont know how relevent but my engine was always bringin up this code after mapping but then i dont have a main cat. andy if ur in essex soon you can try my sensor as it not currently in use to try if you like. either way hope you get it sorted mate

[andywood]

Some good news to start the weekend (i think!)...

Realised that all the temp measurements across the CAT i had been doing were with engine revving at standstill, i.e. no load on engine so not really working that hard. Had a blast in the car this morning to get the CAT definitely nice and hot and then repeated the temp scans. Now shows temp increase across the CAT from approx 270degC on inlet to 360degC on outlet.

Now finally i have some facts that suggest the CAT is doing some work (when up to temperature), this also supports the fact that the post CAT o2 sensor does normally give a steady non-fluctuating output which also suggest the CAT is doing something to the gases after they have passed the manifold sensors.

Next thing now has to be to replace the post CAT o2 sensor, all fingers are crossed that the diagnostic is just being flagged by that occasional random drop to 0.0v on the sensor from a steady reading.
(If that is the case I will owe Aaron a few beers!!)


Andy

[Wabbitkilla]

Actually sounds like typical sports cat behaviour.
Temp readings sound as to be expected from a working catalyst.
O2 sensor sounds like it's doing its job.

What I've found with sports cats is that when you're not driving or putting the engine under a fair amount of load they're not doing a terrible lot, it takes effort to get them up to operating temperature. Because they're invariably smaller than the regular catalyst they tend not to hold onto their heat as well. So you get the occasional dip below what the ecu expects of a working catalyst. Therefore you get P0420 when it's occurred three times.

Proof of the pudding is when I get my car MOT'd, we really need to get some heat built up so it operates fully.

Simplest course of action is to put a bung extender in for the third O2 sensor to fool it into thinking the gases are cleaner than they are. Other options are to replacee it with an electronic gizmo that mimics its behaviour, or replace the ecu with an Apexi PFC.

[andywood]

Cheers for all that, had been considering 'sport CAT behaviour' as part of my investigations but had ruled it out as:
1. Never had a single problem since the turbo and sports CAT was fitted 1 year ago (6000miles) - would have expected it sooner if there was just a sensitivity of the diagnostic to the new sports CAT.
2. A number of other cars running exactly the same set-up with no issues (as far as I am aware that is).
3. Only seems to flag on a long steady run, this is most likely when the entry conditions for the diagnostic are satisfied, but the CAT will be nice and warm at this time as motorway cruising so suspected something else. (Now seems that this might be the post CAT sensor occasionally showing a 0.0v which might flag the diagnostic.)
4. Have done loads of journeys in it over the last couple of weeks (i.e. >3) so if there was a real issue with the CAT performance I am guessing that I should be seeing the p0420 flagged more often. (unless the p0420 only gets it entry conditions satisfied on long steady runs).

Gonna get me a new post CAT sensor this week and then have a long run next weekend - that will hopefully confirm that it was the sensor (if the OBD tool shows that the CAT check has been completed but no MIL) or will show that it is just a sensitivity issue due to the use of the sports CAT (in which case I will investigate further the options you mention above).

Cheers

Andy

[Mike_V]

Good news Andy,pleased you've got it pretty much sussed.

[andywood]

Nooooo, don't go saying that, I am just trying to stay optimistic, but you may have just jinxed it all now   s:wink: :wink: s:wink:  


Andy.

[andywood]

So, new o2 sensor fitted and light came back on today, still p0420, that's not the cause then but all is not bad news!

Took the car for an MOT emissions test straight after a good blast to get some good temperature into the CAT, test was passed okm but there was a clear temperature sensitivity of the sports CAT when it cooled after sitting idling.

Thinking about it all a little more, the p0420 only comes on every now and then, always seems to be on or just after a steady cruise. The CAT is clearly doing it's job as is the diagnostic and o2 sensor, i think that when i am steadily cruising with little load on the engine the air flow over the CAT is enough to cool it to the point where it starts its strange 'cool behaviour', the diagnostic sees this and flags p0420 thinking that the CAT (it thinks is stock) is not doing its job.

Guy in garage reckoned that he could hear 'plug fluffing' when held at steady 3000 rpm, so gonna put some new plugs in just incase things aren't burning up properly and causing strange happenings for the o2 sensors. Never had a misfire code flagged though.

So conclusions are, passes MOT so no need to change CAT. P0420 may occasionally flag but will be interesting to see if this could be resolved by doing something like heat-wrapping the CAT to avoid excessive cooling on steady drives, or maybe just need to drive it hard all the time!
Is pretty common it seems to get p0420 with sports CATs or de-CATs - can't use an anti-fouler thing though due to lay out of exhaust, so if can't be bothered to reset the code when it occasionally flags then may try heat-wrapping the CAT or getting a o2 sensor simulator or something.

Pretty relieved that everything is actually all ok, just a little sensitive   s:wink: :wink: s:wink:  ........


Andy

[andywood]

Oh, forgot to mention, the other solution which can apparently be done is a little 'tweak' on the Unichip Q which means that the p0420 diagnostic will always see a good result.
This may be the best solution because as long as the CAT is good enough to pass MOT there is no real need for the diagnostic anyway!

Andy

[nathanMR2]

I stand to be very much corrected but isnt heat wrapping the cat a big no no. Im sure ive read everywhere that you shouldnt do it?

[andywood]

An update for those interested, seem to have worked out and fixed the issue   s:wink: :wink: s:wink:   (that should be enough to tempt fate for the light to come back on tomorrow!!)

Was thinking long and hard about the driving situation when the code is flagged; based on an assumption that everything else on engine is running fine (as it does seem to be under all checks performed), o2 sensor is fine, CAT does it's job when at temperature, spark plugs replaced, MAF checked, no other codes etc....

Fault always seemed to flag immediately after long lift-off/overrun, so I started to think what may be happening at this time. Long story short-ish.....

Steady driving + boost = lots of oil/fuel vapour in crank-case; lift-off at high-ish rpm = lot of engine motoring with no firing and closed throttle; closed throttle = vacuum in intake; PCV does it's job and leaks crank-case gas to intake manifold; however normally the rear breather is also in vacuum but in my case is to atmos pressure in intake duct and also momentarily sees positive pressure from turbo spinning back on itself at lift-off (as no dump-valve) creating increased flow 'through' engine; higher than normal influx of HC's/oil vapour to engine which is mainly unburnt and is too much for o2 sensors and CAT to handle in normal way; pre and post CAT o2 sensors see similar rich/lean wave = flag p0420.
Essentially therefore my logic was that the engine effectively breathes through itself on closed throttle overrun and sucks crank case air into intake via breather system (path of least resistance as IACV and closed throttle won't leak all that is needed).

The solution - fit Krankvents (Thanks Aaron   s:wink: :wink: s:wink:  ) = no more MIL   s:D s:D  
(The rear breather can now only allow air flow out of engine, Krankvents will also help to protect the PCV from any boost leak into crankcase, although there was no evidence of this in my case).

Light had been coming on religiously every third journey, has now been good all weekend over numerous journeys and in same driving style that would normally help highlight the fault.

Will continue to test and monitor this week, but pretty happy with myself at the mo that have managed to work this down to root-cause. A simple fix of eliminating 3rd o2 sensor or using anti-fouler would just have masked the issue.
Appreciate that all engines are different and breathe/blow-by in different ways, but this may help someone else in the future.....


Andy.

[dcod]

Thanks for the trouble and effort in sharing this information. I've noticed quite a few problem threads are not updated with the solution. (think I've done it myself  s:( s:(  )