P0420 2006 Honda civic si

[Nankumar Seewdat]

Hey guys, I have a 2006 Honda civic si with about 280000 kms on it. The check engine light comes on every now and again reading p0420. I’ll give you a break down of what I’ve done so far.
– first time CEL came on I changed the rear O2 sensor with denso unit and fixed exhaust leak before cat. Only p0420 code shown

– light came on again this time with a misfire in cylinder 1 (along with p0420). Changed all spark plugs and coil in cylinder 1

– light comes on again with knock sensor code and p0420. Replaced knock sensor (sensor was broken in half so I can confirm it was a bad sensor)

– light stayed off for about 2500km before coming back with a misfire in cylinder one and p0420. Cleared it hoping it might be a fluke

– light is back on with p0420 no other code.

Now I’ve noticed that the light only seems to come on when it’s cold as during the summer i don’t remember it coming on. I bought a obd2 scanner (those WiFi ones) and this is the freeze frame data it provides . The thing that confuses me is the part where it says DTC 4 MIL off. But when I scan it only one code shows. Any help is appreciated, I’m just trying to confirm that it might be a bad cat before I pull the trigger.

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[Billy]

An infrared thermometer is very helpful here, they’re like $25 on Amazon. The downstream side of the cat should be about 50 deg C hotter than the upstream side once the car is warmed up.

[Nankumar Seewdat]

Ok thanks, I will look into that once I get some free time. The obd scanner also provides me the temp of the cat, showing that (from a cold start and warming up, not driving around at operating temperature) it was about 560 degrees Celsius and still climbing, according to etcg video on the cat it should be between 650-870 Celsius, does that sound like sufficient temperatures?

[Billy]

Presumably you’re getting that reading from the lower O2 sensor; it will heat up independently of the cat and will not necessarily provide an accurate indication of the cat’s temperature. More to the point, the difference between the 2 ends of the cat is what’s critical here; a single point doesn’t tell you enough.

[Nankumar Seewdat]

I apologise for the long delay in my reply, but i’ve been getting things done to the car and haven’t had a chance to look at the temps yet. I remember looking at ETCG’s video on how to check if i have a clogged exhaust by checking vacuum at idle and then under load. The number’s ive gotten are around 22 hg and it climbs a little bit under acceleration (got these numbers from the scanner not an actual vacuum port). I will post the temps as soon as i get a chance, thanks for all the help!

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