Mark P.

Hello All. I was hoping to get some advice from all the Honda Experts out there. I've got a 2002 Honda CRV with 155K miles. In the last month I started getting the dreaded P0420 code. There was no visible damage to the existing Catalytic Converter (holes, dents, rust, etc) so nothing suggesting exterior physical damage as the problem. I previously had a 2004 CRV and experienced an O2 specific code where a new O2 sensor fixed the problem. So, since I didn't get an O2 specific code on this vehicle I thought maybe the Cat had in fact gone bad. I bought what I believe is a pretty good aftermarket OEM compatible replacement CAT from Magnaflow. Installed with no problems and then still got the code. Further analysis of my O2 signal voltage readings show something is not quite right. My upstream O2 reading when in closed loop and at 2,000 RPMs is only showing voltage fluctuations between .26 and .29V so it definitely doesn't seem to be giving normal rich/lean fluctuations. The downstream voltage readings fluctuate from .1 to .8V suggesting I'm not getting a good fuel mixture burn in the .5V range. My freeze frame info when the code was captured shows Short Term Fuel Trim 3 and Long Term Fuel Trim 3 at -100.6%. My thought is that the upstream O2 is consistently giving a lean output signal to the ECM in the .26 volt range resulting in an overly rich condition when the ECM adds fuel based on the this signal. Then if the O2 reading never actually fluctuates from the lean signal, a rich condition is occurring and throwing the code. I'm trying to determine if I may have a lazy O2 that's not bad enough to actually throw an O2 code or if there may be some other condition causing the O2 sensor to give a low but possibly correct reading such as vaccum leak, injector problem, etc. The car runs fine with no symptoms other than mileage being lower than normal.


Thanks in advance for any thoughts out there. I feel like replacing what I think might be a lazy upstream O2 sensor but I thought maybe someone can provide some insight based on the data I've gathered. Thanks in advance!

Turbokraft

While monitoring the upstrm snsr rapidly pump the throttle while in gear (A/T trans only), this action will produce a momentary rich condition, the snsr should reflect this as a voltage rise to about .85v. If it does not replace it and re-test.

Mark P.

Thanks, I'll give this a try Thursday when I get back to town!

Mark P.

I was able to perform the test today on the upstream sensor. With the car already warmed up I rapidly pumped the throttle and did get a voltage rise up into the .8+v range. So, goosing the throttle did show that the O2 sensor displays what looks like a full range in varied throttle positions. Looks like it didn't support my guess of a possible lazy sensor. I guess that previous reading just supported a test done at a constant RPM value. Thank you for suggesting this test, you probably saved me from replacing a sensor that is still good. Plenty of voltage range on the downstream sensor also when same test is performed. Now I'm back to wondering why I'm getting the P0420 code with two different cats. Any ideas on where I should focus next? I greatly appreciate your help!

Turbokraft

If you have access to a scan tool monitor the dnstrm vs upstrm O2, if they "mirror" one another the cat has failed or is simply under engineered. Not at all uncommon for an aftermarket converter to not comply. The required precious metals would drive the price near an OE converter if it were to last like an OE. Check into a Walker brand catcon, have had reasonable success with them on Civics and Honda minivans.

Mark P.

Thank you for the reply. Yes, since my scan tool is not bi-directional and can't provide live data, I've ordered some OBDII software for my laptop and the OBDII/USB cord. I'll check both O2s in real time to see if they're mirroring each other and be able to get a better feel for pattern waves on the upstream sensor. Since the cat I bought came with a 5 year warranty I thought maybe it would be alright. Thanks for the tip on Walker, I'll keep that in mind after further diagnosis plays out. Thanks again!

Mark P.

Update: My OBDII software showed the O2 sensors were mirroring each other. I then checked my F150 for the heck of it and it showed what you'd hope to see--- primary fluctuation with a steady/flat secondary. The parts house swapped the CAT for the same model on the off chance the first was faulty. I didn't feel confident though as the O2 readings after install seemed very similar to the first one and the sensors seemed to mirror each other. I thought I was in luck though after three days of driving and still no code since the first replacement CAT flagged a code the day after install. Then today my wife says the light is on again. Damn. I guess my next step will be to return the Magnaflow and try to find a Walker. The local Honda dealer quoted me $1,600 just for the CAT. I figure I can try another aftermarket before I even consider paying that price. I may try to find a used one although I know that will be a challenge as two junkyards already told me they won't sell used CATs. Thanks for your help! I appreciate all the advice I can get on this one, it's wearing me down!

Turbokraft

Seek a cat that is Cali compliant be it a Walker or otherwise. That state doesn't fool around and if a company sells there they have to be on their game. A low mileage used would be best.

Mark P.

Thanks for the Walker recommendation. However, I am happy to report that when I actually checked the code after my wife said the light had come back on, it was actually a P1166 and P1167 for the primary O2 Air/Fuel sensor heater. I confirmed voltage to the heater wire and then found the heater element open when doing a continuity/resistance test. Based on those tests I replaced the primary O2 with a Denso brand and haven't had any other problems. P0420 never did come back after installing the replacement Magnaflow CAT so hopefully I'll remain lucky with that one. I will pursue the walker brand CAT though if I ever need another one. Thanks for all the great advice!

Mark P.

It had been a couple years since I had started this post but I just wanted to follow up in case this extra info might help anyone working on that dreaded P0420 code. The code did come back on this car and after some more thinking, and troubleshooting I did a very close inspection of all intake components as I was wondering if there could be an air leak allowing extra unmetered air into the mixture and bingo! On the old brittle plastic tube that connects the air box to the intake, the entire tube had a split underneath it which was completely invisible from a normal visual inspection. The tube had split right alongside the clamp that held the tube to the intake manifold. What likely happened was when I had moved the air box cover to access the air filter the old brittle plastic tube split on the bottom and I never realized it. Replacement with a new flexible air intake tube finally did the trick on this P0420 code. Every time the engine was under any decent load tons of unmetered air was coming in. After replacing the air intake tube I ended up re-installing my original CAT with no more issues. So if anyone out there wants a good deal on a magna flow cat, let me know!

motoxxxman

That means the issue was the cat, which is what a p0420 code is for: low cat efficiency. Replacing the cat solved the problem. Replacing the intake tube had nothing to do with it at all.
The intake tube had absolutely nothing to do with it. And preludes don't have MAF sensors, so there is no metering of air going on with the intake tube. The split in your intake tube causes absolutely no problems with anything except possibly allowing dirty air into the motor. Nothing else.

Mark P.

Hello Sir thank you for this info. I am once again using the original CAT though without the P0420 code reappearing. It was the original CAT that was installed when the code first started. The code did persist with the original and two aftermarket cats until I replaced the air intake tube. At that time the code went away permanently with the aftermarket CAT and stayed off when I went back to the original CAT. This issue was with a CRV though and not a prelude so it did have a MAF sensor in the tube (upstream of where the air leak was). I assumed this info would make a difference but always happy to learn from the community out there. Thanks!

motoxxxman

my mistake, I thought I was in the prelude section for some reason.