I have a 2002 Honda Civic Si that recently threw a check engine light. I had my car scanned and it came back with a P0420 code. I replaced the cat with a California legal aftermarket catalytic converter and the code came back after a few months. I replaced the downstream O2 sensor using the spark plug defouler trick and replaced my spark plugs but I still get the code. I'm at a loss. I'm going up to Aamco to get my fuel system cleaned and see if that does anything. Any ideas?

 

I has same exact problem. The defouler trick worked for me. I screwed the 2 extensions together so that the plug is a good half inch away from exhaust flow. If you really want it to go away I have heard you can just zip tie the sensor out of the way. Its just there to see that the cat is working.

 

yeah, i used the defouler trick as well using two spark plug defoulers. still no joy...

 

have you tried clearing the code and see it it comes back?

 

i cleared the code by disconnecting the negative battery terminal for about thirty minutes. check engine light was off for about ten seconds before coming back on.

 

That should be your sign then, p0420 is a 2-trip logic code. if the monitor had been cleared it would take 2 solid drive cycles before the code would re-appear. take it to autozone or somebody that has a obd2 scanner and have them actually clear the codes out.

 

I see. OK, I'll take it to get it cleared first thing tomorrow. I'll post the results tomorrow.

 

Just want to point out a great comment. Very knowledgeable answer

 

thanks bud

 

Is this logic true for SRS codes as well? I have a pesky SRS light which won't go away after resetting the battery, and Hondata doesn't let you clear SRS codes.

 

No, SRS codes are set immediately. Hondata can't reset SRS codes because they managed in a seperate SRS/airbag computer.

 

Well, I went up to Aamco to get the code cleared but turns out the P0420 code is gone but now I have P1166 and P1167 codes. I checked my fuses and nothing is blown. I heard this code is for a bad downstream O2 sensor. As stated earlier, I bought a brand new O2 sensor- Denso 234-4122. Do I need to go OEM with my sensor and should I replace both? Both OEM sensors are around $200. I went the cheap route and got my Denso for $40. Thanks for the help.

 

did you have to splice in the wiring for the denso? ive never personally had an non-oem AF sensor operate properly and highly recommend oem parts. (and yes im aware denso probably made the original AF sensor.. but a generic doesn't always have the same impedance/range as a stock sensor.)

there is usually a AF heater fuse under the hood and if you havent checked it yet, take a look at it first.

 

No splicing required- it plugged right in. I also checked my heater fuse (green box) and it was fine.

 

Been doing some research- from what I read, it's the primary (upstream) sensor that is bad when this happens. I just replaced my downstream sensor to fix the P0420 and now this comes right after I do my install. Is something wrong here? I really just want to order both OEM sensors and be done with it but I hate to spend money like if it's not needed.

 

Thanks for the write up. Definitely helped my situation as well.

 

troybmm, there are generally 2 common code sets for af/o2 sensors. one set of codes is related to sensor range or malfunction, regarding what oxygen level the sensors are reporting to the ECU. the second set of codes are related to the af/o2 heater. the purpose of the heater is to warm the sensor to the proper temp for it to read accurately.

i've never personally seen an upstream sensor with a heater code affect a downstream sensor.

that being said, looking at this obd2 code chart:
http://www.obd-codes.com/trouble_codes/honda/

your p1166 and p1167 are both pertaining to the b1s1 sensor which is the most upstream sensor in the vehicle. and both codes are related to the heater circuit.

my advice would be to replace the b1s1 oxygen sensor with an OEM honda one and leave the rear sensor alone unless you start having range/heater codes from it.

 

^agreed. I had troubles with an Autozone O2 sensor myself and bit the bullet, went OEM and had no more issues. But this was on a GSR.