Hello All,

Today my check engine light came on after I tried using the AC in the car for the 1st time in a year. The code I got was a PO420. I went to auto zone to have them read and I got a receipt that says A/F Ratio concern, Air leak near HO2S, Ignition system concern, Faulty Catalytic converter. I replaced the catalytic converter and all O2 sensors a year and a half ago. The car has 137xxx miles. Is the check engine light related to me using the A/C? Or could my O2 sensors fail or catalytic converter gone bad that fast? If not how do I check my air fuel ratio or determine if I have an air leak? Please help I really dont want to buy a new catalytic converter again. If you need anymore please let me know.

Thank you

 

I had the same code on my 98 ex, I removed old cat since it was just bolted and the honeycomb inside was melted. I purchased a used car that look in good shape in craigslist for 10.00 and all good for 4 months now... My friend had the same problem, replaced cat a year later went bad again becaused it was a cheap one purchased from a local muffler shop...

 

Okay Thank you not what I was hoping to hear but I'll get under there and check.

Also do you think it could it be a bad fuel filter. I got my oil changed at Jiffy lube last week and they said it needed that the filter needs to be changed. Anyone one think that maybe my problem.

When the car is idling the rpms go up to 1800 then drops to also 0 and then doesnt go over 1000. When the car was fine it would idle at 1200.

If you need anymore info please let me know. Thank you again.

 

My 2000 EX idles at almost 0 when I stop at a redlight and has only shut off once when it does that. My thought was I might need to replace the fuel filter or run some clean through the injectors.

 

Was the catalytic converter an OEM one or an aftermarket one? I just replaced mine with a Dorman aftermarket one and 2 weeks later, I got a CEL. I cleared the CEL and it hasn't repeated. (Though the car just got wrecked and hasn't driven in a week.) It's possible that the header itself is cracked, which is throwing off the primary O2 sensor's reading.

I would recommend clearing all of the codes and then see which ones come back up after driving for a bit.

If I'm correct, pretty much all Hondas idle high at around 1800 and then idle low around 800.

[edit] lol, had too many threads open at once, got myself confused

 

These problems get expensive quick. You need to check your exhaust manifold gaskets, headers, A-pipe, and at the Cat.


My 2000 si cat threw a p0420 at 120,000mi. I replaced the OEM cat with a bolt on Magnaflow, new gaskets, and replaced the secondary O2 sensor with an NTK. 6000 mi later got the P0420 code again, so I got a second Magnaflow cat for free under warranty and replaced the primary O2 sensor with a new NTK, and replaced a suspect gasket where the a-pipe and manifold meet. 6000 mi later the primary O2 sensor failed, so I replaced it with a Bosch. All has been fine now for over 10,000mi. We'll see..... Cant say exactly what the problem was, but I do burn about 1qt/3000mi of oil.

 

Well thank you guys for all the help the cat was not aftermarket. And o2 sensors where bosch. I reset the code and went to autozone to get a fuel filter this a.m. I didn't throw the check engine light back on after I reset it though. But on my way there I was in 2nd and it bogged down. I had my foot on the gas and the rpms dropped by 500 from 3k to 2500 I lifted my foot off the gas and took it out of gear put it back and second and stepped on the gas again. Didn't have that problem again. But there is something wrong. My more immediate problem is now I can't get the bottom bolt off the fuel filter I've tried everything vice grips PB bluster torque on it And nothing but some busted knuckles. I am done for today. I dont want to break anything while I am mad. If you have any tricks to get that bolt off let me know. And while she is stuck there I'll get her in the air and see if I notice any cracks or loose wires.

 

97-00 EX has a different exhaust system with a more traditional cat underneath the car. The DX, LX, etc. can be refitted to that type of system.

If you're seeing your cat melt down, that's because the engine is running rich and/or misfiring and putting raw gas into the exhaust. This gas then burns in the cat as it is designed to do, but it is only intended to handle typical small amounts that would otherwise be HC pollution going out the tailpipe. Too much raw fuel going into a cat overheats it.

 

Under no circumstances should anyone at Jiffy Lube, Valvoline, etc. be trusted.

The car bogging down under load could be related to the oxygen sensors, though (a bad sensor will throw a P0420 just as quickly as a bad converter). Is the car idling fine (ie, doesn't try to stall with the clutch open or the trans. in neutral)? It's been a while since I've been in a civic, but I believe the first mark on the tachometer is 500rpm; idle at operating temperature should be around 600-800rpm under no load (ie, headlights off, A/C off, etc). and somewhere around 800 (I forget exactly what it should be) under load.

Anywhoo, the P0420 is pretty much what happens when the ECM checks the front O2 and the rear O2 and they're out of whack. If viewed in a waveform, the rear O2 should directly mimic the front O2's voltage, only on a smaller scale. If there's sudden drops/spikes in the rear O2 voltage during idling or steady throttle under load for two (I believe consecutive) driving cycles then the ECM sets the P0420. Technically a wiring issue could also cause a P0420, but I have yet to see that actually happen on any car.

Checking over the exhaust system (bolts tight, no holes or leaks) isn't a bad idea, either, but here's the thing about those checks... intake/vacuum leaks, exhaust leaks, low fuel pressure, leaking injectors, etc should set a rich/lean code before it sets a P0420. I'm not saying ECMs always set the codes they should set, but in theory that's how it should work.

As far as the fuel filter bolt, is it seized or just really tight? If it's seized you might be SOL... just coat it in penetrating oil and be careful not to break the bolt. If it's just really tight (and you're using a combination wrench), try using the wrench-on-wrench trick. Put the box end of whatever wrench you're using on the bolt head and lock the box end of another wrench onto the open end of the first wrench and KERBLAM! More torque on the fastener. (Judging from the "kerblam" I'd say my morning coffee's kicking in )

(and mk378, wouldn't a lean mixture also melt a converter after a while? It's been a while since I've had a course on engine theory, but don't lean mixtures burn hotter than stoich or rich mixtures?)