Hey all, I've been fighting this P0420 cold start issue for a long time on our 2000 civic.
Late 2019 I replaced the exhaust manifold & upstream O2 sensor. (pictures of parts attached)
The car has cold start problems. Plugs look good (other than they were covered in gas this morning after it wouldn't start)
I checked the upstream O2 sensor & it had 13 ohms, sounds like 10-20ohms is within specs.
Hooked up my ODB sensor & drove it a bit. Upstream O2 seems to be within spec but the downstream appears not to be. It's supposed to be around 0.5 volts & pretty consistent but mine fluctuates.
I checked the temp before & after my cat. It was hard to get a good reading on the inlet side but it appeared to be close to the outlet side of roughly 350 degrees.
Any idea what my problem is? I'm leaning towards the cat being bad.

 

I generally replace o2 in pairs as it seems one older is a lazier than a new one.
also, how do your plugs look?

 

Makes sense. Think it's worth trying to replace the downstream O2 then?
Plugs are okay, but worn. They only have about 2 years on them.

 

Well I have both situations force that code. One was either bad plug or wire, cant recall. And trying to replace only one 02.
I see it as a tune up never hurts, and I would consider another o2 is cheaper than a cat.

 

After digging into all of this further, here's my update.
ATP Automotive exhaust manifold & catalytic converter was replaced about 2 years ago.
I did a Bosche primary O2 sensor also.
After testing a bunch of things today, it looks like my cat is bad. The primary O2 sensor stayed within 0-1V as I drove it and the secondary O2 stayed as stable as possible around 0.5-0.7V.
Checked the temp on the cat and it's around 600 on the inlet & 300 at the outlet.

I don't know if this will fix my cold starting problem, or if this was a result of the cold starting problem? The car runs great otherwise.

I'm about ready to junk the car because I'm so sick of tracking this problem down. But it starts, runs and drives beautiful. Other than when it's cold out...

 

The HELM manual lists the probably causes for the P0420 as being either the 1) Catalytic Converter or 2) the secondary Heated O2 sensor.

The repair procedure basically says to reset the code, test drive, and replace the Cat. if the code comes back.

But, the note above the test procedure also says it could be due to a leak in the exhaust. If you've a leak, it could be causing an erroneous signal to the secondary O2 sensor.

If you plan on keeping the car, I'd recommend finding a copy of the manual (digital or physical on ebay).

 

Also, regarding your cold start issue. You should be using NGK spark plugs. And, you should test the spark strength from the plug gap all the way back to the distributor components to make sure your getting the best spark,