I have a 2003 Civic Ex. it has 100k miles on it. here is some backround on it. The car was given to me. I was told by the individual that the check engine light has been on for 2 years but they never checked it. I went and got it tested at auto zone and got an error code of P0420 saying Catalyst System Efficiency Below Threshold (Bank 1). I replaced the first o2 sensor. the light went off, but came back on. I checked it again at autozone and received the same code. Is it my second o2 sensor or the catalytic converter ? I need to fix this so i can pass Arizona emissions.

Another thing is that during the summer, once the car is running for a while the AC will alternate between blowing warm and cold. I dont know if this matters but it may be helpful in diagnosing this problem.

 

Basically, P0420 is only thrown when the catalytic converter is bad.

For the A/C, check whether the refrigerant is low in the system.

 

Running into the same problem, except oddly with the warm/cold blowing of the a/c, the temperature gauge in my car would go to the top from time to time. I am still trying to find someone who can figure this out. I was told my P0420 was not my cat. And the overheat stopped for a while but the code is still there.

 

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Resolve the overheating first since that's literally life or death for your engine. Assuming the coolant level is OK (check in the radiator, not just in the reservoir tank), the next thing is usually to just replace the thermostat. If it only overheats when in traffic, never on the highway, the radiator fans may not be working like they should.

Air conditioner working intermittently is often the compressor clutch gap, or since it's a 2003 model a lot of compressor thermal switches went flaky. When it is blowing warm, pull over and check if the condenser fans are running and if the compressor is engaged. Also push the A/C button off then back on and note if the idle speed increases when you turn it on.

 

i have a 99 civic ex. i had the same problem. i spent 500.00 for 2 new 02 sensors and a new catalytic conv. the light came back on! the car had low idle (randomly) it also blew soot out the tailpipe, and fouled out my spark plugs after a short time(about 2 months.) THE FIX, i read about someone on this forum removing the "rear" 02 sen. &putting a spark plug non-fouler in then put the 02 sen. back in.i had to drill out the non-fouler to 1/2" but after that, i was done in about 30min. this solved the problem!! the car runs great, no engine light, no more problems of any kind.AND, i'm getting 3mpg better! NOTE; the post says to use "2" non-foulers. i only used one. you can find this post by googling "02 sensor non-fouler"

 

You probably didn't fix the problem, but you did get rid of the code.

 

Ron J,
Sir, I know this post is old but I am having the same P0420 code. I wrote a lengthy post on my problem the other day on this forum. But, I did notice (my wife as well) that I smell exhaust much more than normal in the past few days since the MIL light came on and I got this sode read. It is the only code. My Civic is a stock 03 EX with 150K miles, which I have taken very good care of I might add. Before I go and buy a new cat (I've replaced both 02 sensors, the down stream the other day when this problem reared its ugly head) where would I most likely find an exhaust leak? And not that it matters either but this MIL light came on a few miles after I left the garage to get my altenator changed..... coincidence? Thank you for your time!

Gene
US Navy Chief (ret)

 

I have this same problem, with an additional one: loss of power. I too have the dreaded P0420 code in CA. Only recently, however did I realize the CEL bulb was burned out and I got the code when I replaced it.

I have been smelling exhaust for many months now and suspect the CAT has been failing for awhile now. I also notice a loss of power in lower RPMs, and this loss of power seems to be getting worse and worse with time. Yesterday, I barely made it up the freeway on ramp in 3rd gear. I suspect that if I didn't have RAM air intake, I would feel this loss of power across the board.

I had to order an aftermarket OBDII CAT for $250 because Honda wants $1200 to install the OEM cat.

I would assume that in my case, the exhaust smell is not from an exhaust leak, but more or less from exhaust being backed up by the failing and clogging CAT.

If you've replaced both O2 sensors and still have the P0240, your CAT is most likely bad, especially if you notice any lack of power like I do.

 

More than likely the Catalytic Converter, or the exhaust could have a hole. I have a '97 Civic, and I took the heat shield off of the exhaust manifold, I saw that someone had tried to weld back together the "Y" section of the manifold. You could check if there's a significant exhaust leak... go buy some Seafoam, follow the directions to get it into your engine: make sure the engine is warm, then remove a vacuum hose while the engine is running-I used the brake booster. SLOWLY pour about 1/3 to 1/2 the bottle of Seafoam into the engine while it is still idling. Then turn off the car, let it sit for about 5 minutes, and turn it back on. When you turn it back on you will see a TON of white smoke coming out of the exhaust. I was able to tell that my exhaust was leaking, without a doubt, because half of the thick white smoke was coming from the exhaust manifold. After I replaced the manifold and CAT with one I found on eBay for $170, everything was better! No more leaks or CEL.