P0420 – how to solve once and for all?

[davey]

I’m sorry to make a re-post, I have made a thread about this a while back.

I have a 2000 Ford Fiesta with a 1242cc engine (1.25 officially). I have continually had the check engine light come on, and my scan tool says “P0420 catalyst efficiency below threshold”. I have seen the ETCG videos about the code/bad cat etc. I’m not sure how to diagnose my cat is at fault. Firstly i replaced my downstream (post-cat) o2 sensor with a brand new NTK part. The code returned.

I used the live data on my new scan tool and followed the ETCG o2 sensor video for information. I bought up custom live data set of the two o2 sensors. The upstream sensor was fluctuating anywhere from 0.100V to 0.900V as per the video, and the downstream sensor was dead steady at 0.495V and did not spike or fluctuate, regardless of RPM.

I’m not sure how the code is being stored, if the sensor is reading bang on voltage?? Even with the check engine light off, if i plug my scan tool in it says the code is pending.

Really confused by this one guys.

The last MOT test i had in February of 2013 the emissions test came back just fine so the cat must be OK. Right?

The two logical options i have left is either a replacement cat, or source an ECU from a slightly earlier year (of the same model of car) without the post-cat sensor.

Would really appreciate some advice!

Topic

[Gumpy Guss]

If you passed the emissions test, that should be your gold standard right there.

The O2 being steady at the outlet is telling you that the cat is doing something.

Also from Eric’s video, he didn’t IMHO quite stress enough the significance and the importance of finding that the cat outlet temp is much higher than the inlet temp– that tells you that the cat is working, it’s cooking the excess hydrocarbons and it’s not poisoned by lead or carbon or something else.

So: Hot cat, good cat.

There could still be something wrong with the exact levels of O2 being reported at intake or outlet and that could generate an error code. I don’t think anyone knows exactly what criteria the ECU uses to decide that things are enough off kilter to justify this error code. Did you try changing the upstream O2 sensor? Perhaps the ECU is worried about the in/out ratio rather than just the result.

Another possibility– maybe your cat was just having a bad day, or you were driving in such a way as to not get the cat warmed up well. Try clearing the codes and then do an “italian tuneup”, no really, go out on a unbusy freeway and drive the speed limit for half an hour. That will not only burn off any carbon in the cat, if you don’t get the error code after that things are probably okay again.

Even if you don’t get the code again, it may have been a warning that your cat is getting a bit inefficient, where just driving slowly pushes it into its inefficient zone. Then you have to decide whether to ignore the error, or replace the cat, or just drive more like a hyper b*stard. What a choice.

[davey]

I have to say, thanks for that, a lot of information there.

I watched the bit of the video about the temperature of the cat and how its hotter after, then before. I was confused by this to be honest as i thought all of the thousands of explosions going on that the manifold would be the absolute hottest.. maybe that is the case with my car actually? The manifold and the cat are a one piece unit. Whether or not that’s true for mt car actually, I can kind of understand how it would be hotter after the cat where the filtration of the different parts of combustion are hotter – or something along those lines, i think.

That’s a good thought actually, the upstream lambda. I did take it out once to see if i could clean it etc, and the threads in the manifold itself seemed to be stripped however the sensor is held in sufficiently IMHO. The car itself runs lovely, better then most of its age actually and especially with 131k approaching 132k miles.

Obviously I cannot say how the car has been driven in the past, but since i have had it i have driven quite carefully and never “raced about” so to speak. I bought and used the car initially for travel to college which was 30 miles each way, per day. This was mostly motorway so i think that would cover the “Italian tuneup” as you put it haha.

Thanks alot for your reply, very detailed and a lot of things to consider.

Out of interest, this is the cat/manifold for my vehicle, unfortunately it’s upside down.

[Gumpy Guss]

Yes, the manifold gas is hot, but there are intentionally some extra unburnt hydrocarbons in the exhaust gases, and the cat has catalysts that promote their burning, so the outlet HAS to be hotter if burning is going on (assuming there’s not much pressure change).

Now it’s interesting that you mention in passing that the first O2 sensor may be loose– that’s one problem that I forgot to mention. If there is a leak there, oxygen can get sucked in during the rebound from the pressure pulses, and that extra oxygen may be confusing the O2 sensor. You might want to consider sealing that up with some muffler sealing compound. The pro’s call it “Mud”, but it’s mostly some very alkaline silicon compound.

[Bill Ogier]

P0420 plagued our 2002 Impala for a year. I tried lots of different things and wasted a lot of time thinking about it. What was the (easy) fix? Spark plugs. Been almost 2 months and code and CEL hasn’t come back.

I feel your pain…best of luck to you.

[EricTheCarGuy]

As pointed out the cat is suppose to be hotter at the outlet than the inlet. I know this confuses you but what takes place inside the cat is a chemical reaction that produces heat. This heat is used to help clean up NOx emissions. No internal reaction, no heat, or not enough heat. The heat actually breaks apart the N and O2 into separate components as explained in the video. If you have an aftermarket cat this could be the cause. It’s been my experience that aftermarket units don’t often work as well as the OE units and the computer reads it as a bad cat. It’s something to be considered anyway.

[Author]

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