i have a 1998 honda accord lx 4 cyl 2.3 vtech. ok here it goes, got the car used and right away i put a cold air intake on it...maby 2 or 3 weeks later the check engine light turned on for code 0420 catalyst below threshold bank 1. So i went on ebay and purchaced a cleared out cat converter http://cgi.ebay.com/ebaymotors/OBX-H...spagenameZWDVW
then i had that installed and the code stayed on. Then i purchased bosch platnum 2 plugs and installed them and noticed the old ones were pretty bad. that didnt do it. Then i purchased a bosch oxygen sensor for before the cat, replaced that and still .... 0420. Car seems to ride like a dogg. Sometimes i have trouble starting it...the needles go up and then down and up again. Sometimes while driving the car completley turns off, the last time i was on the highway and the spedomater gage and rpm gague went down like a quarter of the dial and then right back to the main speed i had it set to on my cruise controll. This code is killing me. Another thing im wondering is if maby the honda isnt working right with that cat converter that i put on. When i take the neg battery cable off the check engine lite turns off, and the car seems to run better for a little while, But it always turns back on , sometimes within 5 10 miles, sometimes not for 400 500 miles. but like i said the car just runs like a dogg, please help !

 

The O2 sensor that monitors the catalytic converter is the second one. It is past the catalytic converter. I'm not sure how the 2nd O2 sensor will respond to an aftermarket cat. Even if this O2 sensor is screwed up, the ECU does not use it to regulate the engine. The ECU reads the first one (the one you replaced) to regulate engine.

I remember reading that the ignition control module (ICM) can cause the needles to act strange. The tachometer gets its signal from the ICM. There is a writeup by Deserthonda in the DIY section on how to test for a bad coil/ICM. The ignition switch is also a posibility. When the starting problem happens, you should check for 12V at the hot wire going to the distributor. I'm not sure which color, but some of the techs will be able to tell you immediately.

 

could the cold air intake have anything to do with the 0420 code?

 

I only like the expensive OEM cat convertors and OEM O2 sensors too, sorry but you may have to go the OEM route. P0420 is for the rear O2 sensor, it is the cat monitor, the sensor may be fine, it probably is and the cat is usually the problem but if the engine is too rich or too lean then the cat cannot do its job correctly but if the front O2 sensor was biased rich or lean then there should be a code for that too.

The power to the coil and the ignitor and to the ECU (partial) is the black/yellow wire. If the tach needle is bouncing during initial starting attempt when the engine is cranking but not actually starting then the ignitor is usually good, the tach signal comes from the ignitor. the tach sould only go up to probably less than 300 RPM while cranking but not actually starting.

You may have a weak ignition coil, you would need to pay for an Amp Ramp scope pattern inspection to see what the shark fin pattern looks like but that takes training, any shop with a scope and an Amp Clamp should understand the pattern.

As mentioned, the P0420 should not effect the drivability of the engine. A scope pattern of the front sensor is in order and a 5 gas readout if you have some extra change to get a real diagnosis.

 

P0420 is a catalitic converter code, NOT an o2 sensor code. Have you reset the ecu after installing the "high flow" cat?

TAKE THOSE BOSCH PLUGS OUT IMMEDIATELY!!! THEY WILL FRY YOUR IGNITION COMPONENTS!!! I AM DEAD SERIOUS. HONDA'S DO NOT LIKE BOSCH PLUGS. THAT IS PROBABLYWHY YOUR CAR IS RUNNING ROUGH.

 

Bosch plat's are the wrong resistance plug type and KILL the ignition coil. This will result in mis-fires causing CEL codes and will eventually strand you somewhere ....... most likely when you are farthest from any help.

 

I have heard of CAI's causing P0420 on other cars, but I've never had this problem myself,...course I've never used a CAI either.

P0420 can be caused by bad catalytic converter. The ECU monitors the aft converter O2 sensor and watches for cross counts (voltage crossing mean value). Too many cross-counts and code P0420 is set. P0420 can be also caused by excess air entering the exhaust from a cracked exhaust manifold or leaking exhaust connection. This causes excess cross counts and triggers the code.

You can have the exhaust monitored for pollutants w/ an exhaust analyzer and verify excess NOX, CO, HC, etc. If the exhaust looks good, look for an exhaust leak.

I used an Eastern after-market catalytic converter on my 99 Subaru Forester which hadP0420, and it's worked fine now for 6 months and the O2 sensor voltage trace looks good. I think there are good aftermarket converters, but there are likely some poor ones also.

good luck

 

just replaced the spark plugs to ngk...and didnt notice a difference. also i am noticing a whistle after around 3000 rpms, is that the vtec kicking in or what?

 

It could be a vacuum leak which may be causing the O2 sensor to be biased and causing the cat to not work properly making your code, I think a test drive with a scanner reading scan data is needed here.

Your noise could also be a bearing squeeling on the engine or an air leak in the intake tube prior to the throttle body, in this case it should not effect anything but I doubt the intake tube will whistle.

 

how do you test for a vaccum leak?