javiertololo

Car: 2002 Honda Civic EX Coupe. 5-speed manual, Apprx 194,000 miles. The car is completely stock. My check engine light came on, took it to AutoZone and got code P0420. Which apparently, by looking at a bunches of forums, it has to do with the o2 sensor, catalytic converter, A/F ratio, and ignition system.

So by reading what it's in the forums, what I need to do is check the upstream o2 sensor to see if it's good or bad. Apparently the tools that I need are a multimeter, and figuring out where to connect it. So far so good, but I don't know where to connect it. I unplugged the o2 sensor and the sensor itself has 4 wires, 2 white, 1 black, 1 gray. Where do I check with the multimeter? Do I have the car running while doing this? What are the readings that I'm supposed to get? I also saw on some posts that I'm supposed to check the wires coming from the car, but it's hard to see what the colors of those wires are. So once I check the sensor and figure out that it's bad, I just replace it, simple solution right?

But if the sensor is good, it might indicate the catalytic converter or the downstream sensor, correct? If so, do I just follow the same instructions to check the 2nd sensor to figure it out? Anyway, I have noticed for a very long time a metal rattling noise coming from my car when it's idling or at low rpms, once it gets going it's fine, so from reading the posts and forums this might also indicate that there's something wrong with the converter? If that's the case, how difficult is it to replace it? Is there any welding involved or just bolt on?

I'm sorry to throw so many questions out there, but I've been looking and I just can't find the answers, or whatever answers I find are not very specific and I may also not be savvy enough to understand what they're saying. So, could somebody explain it to me, just assume I'm stupid and broke. Thanks in advance!!!!!

GriFFIthS

if one of your O2 sensors are "bad" then it will show a different code.
P0420 is Catalyst System Efficiency Below Threshold. basically your Catalytic converter is shot and you'll need to replace it.
Now the cat and exhaust manifold are 1 piece in these civics, and they are all famous for having the manifold crack between #2 & #3 cylinder pipe. If the crack is large enough it would allow air in and could be throwing your o2 sensor off which would explain the P0420 code.
Replace your Cat and you should be fine, no welding, just unbolt the exhasut from the bottom of the cat, unplug both 02 senors, and unbolt cat from blot. you may need a new exhaust gasket and spring bolt kit, but they are cheap so no worries on that part, should be able to get an oem replacement for around $250. dealer will change double and triple that.

Easy way to know if your cat is gone is to remove it and give it a shake, if it rattles its gone and needs replacement.

javiertololo

The light went off on friday morning and I didn't do anything to the car, is that normal? All i did on thursday night is I checked the wires for the o2 sensor, don't know why it turned off??

It Wasn't Me

LX/DX have the catalytic converter built into the exhaust manifold. The OP has an EX, which has two pieces (manifold, and A-pipe / catalytic converter).
As noted, it's easy to replace... usually. The nuts and bolts are usually rusted and seized, often requiring cutting tools (torch, saw, etc) to remove them. No welding, though. Everything bolts back on.


Which sensor? Primary or secondary? (just curious)
Have you ever reset the ECU, then recheck for codes?

It's usually going to be a bad catalytic converter. Sometimes it's just a faulty sensor, or the rare occasion when it's a slipped timing belt. Sometimes it takes the replacement of both sensors and the cat to make the code go away. P0420 is a PITA.



About the noise coming from your catalytic converter. The flanges on the heat shield like to rust and separate, causing the infamous low RPM rattle. Inspect them! Pry on them! Mine looked "okay" but when I pried on the heat shield it separated at the flanges, so I just removed the heat shield all together. No more rattle.

Another possibility (as noted) is that the catalytic converter itself is shot. (which is why P0420 has come up)

jasrams

just got this code today and was worried but i logged on saw it thanks guys

eatpoo

99% it's the cat.

javiertololo

I can't find the video now, but isn't there some kinda liquid that you can use to remove those bolts without having to cut them?


The sensor is the one that's right after the manifold, so that's the upstream sensor. And no, I haven't reset the ECU, but I'll try that, but so far it hasn't come back.

I was gonna take off the cat during the weekend to inspect, but realized that removing anything will just be a PITA and more than likely will need new bolts and whatever else, so decided not to, I'll just wait until I'm able to purchase another one and get it installed, I wish I had enough money to replace the entire exhaust system, my car is way too stock...

It Wasn't Me

Sometimes you can break them loose with some PB Blaster, but I wouldn't hold my breath.


To reset the ECU: Remove fuse #6 from the fuse box under the hood. Wait a minute or so, then reinstall it. With everything electrical turned off (heater, wipers, lights, radio, etc etc) start the car and hold it at 3,000 RPM until the radiator fan comes on. Remove your foot from the throttle and let the car idle for at least five minutes. This is known as the Idle Relearn Procedure and it's to establish idle characteristics.



Hopefully the light stays off. Good luck with it!