Preliminary info: 99 Honda Prelude, VTEC, Automatic (4-speed), 77k miles.

While driving back to work today from lunch, I noticed my CEL came on. Cursing my luck, I pulled into the parking lot, found a paper clip in my car, and used the MIL to pull the error code. After looking it up on troublecodes.net, I found that the error code indicated was: P0420 - Catalyst System Efficiency Below Threshold.

I pray to the gods that my rear O2 sensor bit the dust as I'd much rather replace it than the CAT. Lucky for me, I have a codelink device and laptop handy to hook up to the OBD port. I look at the live data coming from the sensors, and it looks like H02S S2 (rear O2 sensor, I'd say?) has a very pronounced sinusoidal shape, only slightly smaller than the graph for S1. My co-workers say this means two things: 1) the O2 sensor is flaky and reporting bad data, or 2) the O2 sensor is working fine and points to a dead/dieing CAT.

Any suggestions as to how I should proceed? Replace rear O2 sensor, reset ECU, and then wait and see if the same trouble code comes back? Skip straight to buying a new CAT? Both?

Thanks in advance for any help! Also would appreciate good brand names to look for when buying either an O2 sensor or a CAT for my 'lude... all I've heard so far is stay away from Bosch.

 

I was having this same code coming up for a little over a couple weeks. After getting the same 420 code I cleared it to see if it would come back on, it did in about 30 miles. Anyways, I first replaced the rear o2 sensor as it is the cheaper option. CEL came on again so I just bought the cat and returned the o2 sensor (had to clean it up first though).
Problem solved. My prelude is right at 76xxx miles now by the way.

 

GRRRRR....If you'd only use the Search feature, you wouldn't have to ask this question. However, I'm feeling generous all of the sudden, so....

When you receive P0420 with no other codes, 99 times out of 100, it means bad cat. So replace it. However, first check with a Honda dealership. Some of the cats are still under warranty.

/lock

 

OK, what if you have already replaced the CAT, The EGR and both 02 Senders?
Someone told me it was due to the aftermarket CAT that was put on there....is there anywhere I can get a "stock" type CAT that would be a lot cheaper than the dealership????

 

A Magnaflow/Carsound highflow cat should not cause this code. So it sounds like you purchased the wrong aftermarket cat.

 

Unfortunatly no.