A few months back I got a CEL. I ran the code and it came up with P0420 code which is catalyst system efficiency below threshold. I replaced the converter twice, upstream and downstream O2 sensors, coolant temp sensor. And the code still comes up after a day or so. The only upgrades I have are an AEM intake and cat back exhaust. Any help would be greatly appreciated. I'm getting tired of seeing my CEL pop on lol

 

After replacing your cats each time, how long does it take before the code comes back ? Have you tried another ecu ? Also, what kind of car do you have and what engine etc etc

 

The first time after replacing the cat it took about month or two, the second time took only a week or so. It's a 99 prelude 2.2 Vtec. And no I have not tried changing the ecu yet.

 

What type of catalytic converter are you replacing it with, it might not be efficient enough to prevent the CEL. The car may also just need a tune up, plugs, wires, cap, rotor, etc. Make sure you use the OEM Denso brand O2 sensors, not the bosch replacements from the auto parts store. I have also seen ecu failures, so if the CEL pops back up after a few minutes of driving, it could be the ECU. After resetting the ECU, it should take a few driving cycles, sometimes 50+ miles to get a P0420.

 

I just replaced the plugs and wires but it probably could use a new cap ad rotor. I used walker O2's do you think I should try densos? I'm using catco for the converter. It usually takes about 50 miles til the CEL comes on after clearing it.

 

Walker is fine. They do not make their own parts, they rebox and rebadge OEM parts. The Catco cat is likely the culprit. However the vehicle must be in proper fuel control in order for the cat to work, so that must be established first.

 

What are the fuel trims like? Should be in the freeze frame for your code.

 

Short term fuel trim bank 1 = -0.8
Long term fuel trim bank 1 = -10.2

After driving about 500 miles my CEL actually turned off for about 50 miles and now it's back on.

 

I've used catco converters with success on OBD2 cars so that might be ok. Judging by the -10% on the long term fuel trim you are running rich. What about the fuel system, fuel injectors, fuel pressure, etc? Pull out the injectors and send them to RC to get tested and cleaned.

 

The only real way to confirm whether or not it's the cat is to high idle the car at 2000rpm and watch the downstream sensor (post cat) The system uses a narrowband sensor and looks for fluctuations in the voltage reading (it should stay constant). The primary oxygen sensor should fluctuate between .200 to .900 volts...give or take a hundred. The post cat sensor will see a constant flow of oxygen when the converter operates properly.. somewhere between 400-700 millivolts (.400-.700v) If after 2 min of 2000rpm it still fluctuates, try at 2500 for the same amount of time. If the o2 still fluctuates, what I have done is to take the vehicle for 3-4 WOT runs from 2k to redline in 3rd gear and try again. If that doesn't work, the cat is no good. Most aftermarket companies cats are not very good. The Rhodium content is low or lacking.

Try these tests and go from there. Don't replace anything until you prove your faults...your wasting a whole pile of cash for nothing. +/- 16 % is usually within proper perameter for LTFT. If you were out of spec, your ECU would set a code for that.

Let me know how it works out.

 

+-16% is much too high. +-10% is acceptable and even then I'd monitor stft excursions. The cat test you propose is also flawed and may lead to erroneous parts replacement. In order for the test to be valid you would have to know what the PCM parameter for catalysts testing is and whether or not the cat is a high or low OSC cat....

 

Ideal ltft is 0, but around 5% plus or minus is perfectly acceptable for a well running engine. -10 is definitely rich. How are your plugs wires cap rotor and coil? The threshold for a fuel trim cel is over 20% for more than one drive cycle. Running rich will also cause premature cat failure... How dirty is your air filter? May case a 1-2 % change in the fuel trim if it's really plugged.

 

+-10% is acceptable on high mileage vehicles. Particularly older OBD2 software. It is considered proper fuel control, and more importantly ,will allow the cat to function properly.



And no, an air filter,no matter how plugged, will have little to no affect on fuel trim.

 

All the PCM cares about is whether or not the post cat o2 fluctuates. We could get into gas readings pre and post to confirm efficiency for a smog fail, this is strictly for p0420 code. Most parameters on many vehicles is +/- 16%, subject to engineering parameters specified by the programming in the PCM. The purpose of the WOT pulls is to attempt relighting the cat.

 

That is incorrect. There are 3 test procedures in use by all OEMs. Do you know what procedure this Honda is using?
This is also incorrect. There is no OBD2 class vehicle on the road where -+16% LTFTis acceptable. Using that logic will result in an incorrect diagnosis. Learn this now and save yourself some headaches in the future....