Narrowing down P0420

Messages
681
Location
Northern Virginia
Well after 212k, the CEL came on and stayed on for the first time in my Civic's 13 years. I do believe this code came on about 1.5 years ago after getting an oil change. I turned around and had the mechanic pull it, and he reset it to see if it would pop again. Didn't come back until now. Pulled the code with Torque and it was P0420. I also ran the app's "tests" and it said I failed the "Rich to Lean Sensor Switch Time". Minimum value is apparently 2300 and I registered 2119. I've done some digging around Civic forums already for some things to consider. Here is what I think are relevant details so far. I changed my spark plugs and wires about 2 weeks ago. The plugs I pulled didn't look bad, I still have them, so maybe I'll take a closer look. After replacement, I noticed the somewhat rough idle I would get near the end of my 40+ mile one way commute went away. Other than that rough idle, the car has been performing fine since plug replacement. Same as always really. It seems a lot of P0420 codes pop after plugs/wires/distributor replacement. I've probably put about 1200 miles on it since changing the plugs. I got gas last night, filling a nearly empty tank from a Shell I rarely use, but was convenient for where I was stopping on my way home. Gas mileage for the previous tank was on the high end of my usual range. I had a ~6 mile drive afterward going 40-65. A couple stop lights. The code came on this morning about 2-3 miles into my morning commute going highway speed. Performance for my commute today was normal. Fuel consumption so far seems OK, although I did idle for a while in the garage which may throw this tank off a bit. One thing that did slightly concern me was a temperature spike I noticed while sitting in my car idling and playing with Torque tonight. I looked up from Torque to notice the gauge was about 1/8th of the way up the dial higher than normal (if that makes sense), it quickly went back down to normal almost as soon as I noticed it. I popped the hood and thought my overflow tank looked a bit lower than I remembered (which I remember since the place that replaced the radiator had filled it higher than I expected it to be). I'll be keeping an eye on it. When idling I did not notice any smoke out of the tailpipe. Exhaust note sounds fine, so I don't suspect a leak. Oil did not look cloudy on the dipstick. These are the original O2 sensors and catalytic converter. So it could just be time I guess. I just read about way too many "P0420 chases" on various forums, with guys replacing cats, sensors, plugs, and who knows what else trying to clear this code. So I'd like to have narrowed things down before dropping it with the mechanic. Torque seemed to show me the waves of the O2 sensors, which I'll check tomorrow after continuing my crash course in O2 sensors to see what they should look like. Anything else relatively easy to check? Best case I won't be able to get it into a mechanic until next Friday. Too much going on at the office. In the mean time, I might test drive some new cars. O2 sensors and/or hunting down the cause of the code might cross my repair bill line when added to the timing belt service I need done, broken a/c that needs to be fixed, and new oil pan I need. Appreciate the wisdom as always.
 
Messages
13,616
Location
Frisco, TX
"Catalyst System Efficiency Below Threshold" is what P0420 corresponds to on Hondas/Acuras. This means that the rear O2 sensor (post-catalyst) is reporting a failed cat converter. The pre- and post-cat sensors compare their values. If the two report similar values, that means the cat has failed. However, a flaky sensor will sometimes not return the "sensor bad" code as it should. When this happened on my Integra years ago, I removed both O2 sensors and clean the connectors and sensors themselves with electrical cleaner. It's an aerosol you buy at any auto parts store. Ultimately that didn't work and I had to replace the cat. Magnaflow/Carsound makes OEM replacements for about $200 that carry a 100k warranty; an OEM cat is like $900+.
 
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1,115
Location
midwest
How long has the code been on? This code pops up occasionally on my '04 Civic and usually goes away in a couple of days. I only need to have my car inspected every 2 yrs. so I'm not going to replace anything, I can work around the inspections. Dump in a bottle of fuel system cleaner or do an intake cleaning or even just an Italian tune up and it'll probably go away. If it stays on for a month of daily driving then maybe look into replacing the downstream sensor, upstream sensor then cat, in that order. I think cracks in the header can also contribute to this code.
 
Messages
13,616
Location
Frisco, TX
Good point, JB. I would clear the code and see how quickly it returns. Sometimes that class of codes will just kind of randomly come on and then disappear for a year. OBD2 is rather fickle.
 

VicVinegar

Thread starter
Messages
681
Location
Northern Virginia
Thanks for the input. The code has been on for a day. I was going to clear it after I get home today, or maybe even in the parking garage before leaving work since I have the OBD connector in my car now and then would have a 40+ mile ride home to see if it comes back. Is there a downside to Scotty Kilmer's lacquer idea? Could it do more harm than good? Oddly enough his example car is a car I'd consider replacing the Civic with. Amateur question here, do I need to shut the motor off to clear the code or should it be running? I've read it depends on the car, or is that all just nonsense and I can clear it whenever?
 
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Messages
316
Location
MO, USA
The code should be cleared with the ignition on but the engine off. Changing the downstream O2 sensor rarely fixes this code. The code is set when the rear O2 sensor reaches a switching frequency of roughly 80% of the upstream. In essence this says that the cat is not cleaning up the exhaust. A new, faster switching UPSTREAM sensor can sometimes make the code reset. Note that after reset, the Monitor for the cat takes forever to set. Very specific combinations of highway and normal driving are required. The real fix at your mileage is to change both sensors and the cat.
 
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1,731
Location
stl
To me a P040 code means 02 extender and it gone. My Vibe had this code about 4 years ago, did the extender, and it never came back. Took about 5 bucks, and 5 minutes. smile
 
Messages
9,560
Location
Boston, MA
Originally Posted By: partspro
The code should be cleared with the ignition on but the engine off. Changing the downstream O2 sensor rarely fixes this code. The code is set when the rear O2 sensor reaches a switching frequency of roughly 80% of the upstream. In essence this says that the cat is not cleaning up the exhaust. A new, faster switching UPSTREAM sensor can sometimes make the code reset. Note that after reset, the Monitor for the cat takes forever to set. Very specific combinations of highway and normal driving are required. The real fix at your mileage is to change both sensors and the cat.
Changing just the rear sensor can make it worse. I get that code on the Camry used for city driving every so often, a good highway trip makes it turn off on its' own. There is also the "lazy sensor syndrome" to consider.
 
Messages
5,920
Location
Baltimore, Maryland, USA
Originally Posted By: MarkM66
To me a P040 code means 02 extender and it gone. My Vibe had this code about 4 years ago, did the extender, and it never came back. Took about 5 bucks, and 5 minutes. smile
Where can I get an O2 spacer for $5? I thought that most people were using spark plug fowlers and drilling them out.
 
Messages
722
Location
Kansas City, MO
Originally Posted By: zzyzzx
Originally Posted By: MarkM66
To me a P040 code means 02 extender and it gone. My Vibe had this code about 4 years ago, did the extender, and it never came back. Took about 5 bucks, and 5 minutes. smile
Where can I get an O2 spacer for $5? I thought that most people were using spark plug fowlers and drilling them out.
That's the $5 fix
 

VicVinegar

Thread starter
Messages
681
Location
Northern Virginia
Past the actual emission system, is there any relevance to the code and this slight overheating issue I've started to see? I mentioned in the OP the temp spiked a bit while sitting in my garage but sank down on its own. First time I noticed it. Today on the drive home I got stuck in traffic for a while and saw the gauge start to rise. I turned the heat on just to be safe (especially since I sat in that traffic for over an hour) and it came back down to normal I turned the heat off once I was back up to speed and was OK until I hit gridlock again. I didn't need to fire the heat back up, but it did rise a bit while inching along the gridlocked roads for a few minutes. It was only 70 today, so it wasn't exactly hot. When I sat and listened in the garage, the fan is cycling on like it normally does. Still watching the coolant level in the overflow tank, which does look a bit lower than I remembered, but not noticeably different than yesterday. Since getting my radiator replaced maybe 2 months ago, I've subjected it to gridlock in 90+ weather and had no issues until now.
 
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