P 0420 Again! [Archive] - MX-5 Miata Forum (2024)

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Sam

1st May 2009, 21:51

Two new O2 sensors, new catalyst, new coil packs, plugs, wires. I had a slight misfire (occasional missed fire, as seen with a timing light) that wasn't enough to trip a CEL, so that's why the ignition parts. Eastern bolt-in midpipe with catalyst, after about 300 miles, throws a new P0420. cleared, repeat CEL after 40 miles. The post cat sensor is tracking the pre-cat sensor closely, which suggests that the catalyst is indeed a goner. No leaks that I can find. FM trouble tree starts with checking MAF, which I cleaned with MAF cleaner, prior to all this. Intake manifold also cleaned. Pretty much out of ideas.

Before I get after Eastern about the catalyst warrantee (I seriously doubt I had enough of a misfire to kill a catalyst so quickly), where else should I look? any ideas?

Runs fine, mileage as far as I can tell with such a small sample is good, no indication of cat overheating (the carstuff.com shipping lable is still intact, not charred). Just gives a CEL, which based on comparing the sensor outputs, is probably correct; failed catalyst.

?? thoughts?

NB&SV

2nd May 2009, 00:04

What kind of pickup does the timing light have? It's likely that the light is showing a "misfire" if it's missing a pulse every now and then. I've heard about some people having problems with a non OE dealer type cat. Though usually any aftermarket cat will work fine. Did you get an O2 sensor that was pre-wired or one that was universal? It's possible that a universal sensor can have a bad connection and give a false reading.

Chuck75

2nd May 2009, 00:36

Several years ago therre was a discussion of aftermarket cats on the forum. Seems that many of the aftermarkets were two way cats, and the cat needed on the NBs was/is a three way cat, according to the discussion.

dads'mx5

2nd May 2009, 09:38

you aftermarket cat is less restrictive than the stock cat (more flow = more emissions),so you get a failed cat code,even though it has not actually failed.you will have to replace it with a more restrictive Ceramic core (Not Metallic core) cat or install an o2 simulator into the rear o2 sensor to modify the o2 signal to the Ecu.that will eliminate the Po420 code. (recommended for off road use only)*

wojeepster

2nd May 2009, 14:44

a few minutes of misfires is enough to kill a cat, ask me how I know...

OZHAN

2nd May 2009, 16:06

This might sound primitive but have you checked you spark plugs and wires? this happened to me. The spark plug wires get worn from the inside over time or rodents get to them. It might not be visible but it doesnt take a lot for the current to want to travel the path of least resistance = missfire. it also doesnt take alot of missfire to mess up your cat.

hope it helps

Sam

2nd May 2009, 17:46

No new ideas, then.

To catch up, new plugs, new wires. Inductive pickup timing light, no miss on #1, occasional miss (before replacing coil pack) on #2; not enough to trigger CEL. No sign of overheating at all, which is the usual symptom of a killed cat due to a miss. However, the pre-- and post-- cat O2 sensor voltages track, which is a pretty clear indicator that the catalyst, isn't. With a working cat, the pre-cat O2 will be moving pretty quickly from ~0.1 v to ~ 0.9 v as it hunts back and forth between rich and lean; while the post-cat should hang right around 0.5 v, and move very little.

No such thing as two-way/three-way problem, that's one of those internet myths or something perpetuated by a dealer; catalytic converters for engines are either oxidizing (convert CO and HC to CO2 and H2O) or three-way, also convert NOx species to N2 and CO2 with the C & H available from the CO and HC (that's why the mixture has to be so carefully controlled; so the oxidation and reduction reactions balance out). Catalytic converters for automobiles necessarily are 3-way cats, to get all pollutants of interest.

Possible that it's less restrictive than OEM, but thousands of these Eastern cats get sold as OEM replacements with no problems. So, either a barely-detectable miss killed a cat in less than 300 miles (gee, thanks, wojeepster; I feel so good knowing that), or it had a problem to begin with. Any thoughts or success with restoring cats? I'm going to try a toluene wash, which is sometimes successful with industrial catalytic converters, but.....

Another possibility is to install a R-C circuit in the post-cat sensor line to smooth out the voltage changes. For off road use only, of course.

Perhaps Brian Goodwin will have a sale soon.