cylord

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.

Kronn 98SH

Big and bold in the FAQ:

Another DIY ATTS, ABS, & CEL (Check Engine Light) DTC

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1988 Prelude S - victim of an 80mph backflip
1991 Prelude Si - sold
1989 Accord DX - impounded
1998 Prelude SH - totaled on 3/29/12
2006 CRF150F - sold
2006 YZ250F - sold

lviii

I'm not encouraging you to skip the black letter troubleshooting steps . . . but replacing the rear oxygen sensor cleared the p0420 for me (98 SH).

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Current
01 Prelude Base SS (Is that my tranny ticking? )
98 Prelude SH (FGP and loving it! )

Kronn 98SH

^ You're the first I've ever heard of.

__________________
1988 Prelude S - victim of an 80mph backflip
1991 Prelude Si - sold
1989 Accord DX - impounded
1998 Prelude SH - totaled on 3/29/12
2006 CRF150F - sold
2006 YZ250F - sold

Retsiger

Replacing the rear o2 sensor also fixed p0420 for me. Although, I was also pulling error code p0171. (which a new o2 sensor also fixed.)

bouckarooo

i had a p0420 and it was due to a failed cat(all the platinum catalyst agent in side the cat had burned away), issue was solved by installing a new cat.(mind you i used the old o2 sensor, still took away my issue).

istn the p0420 -catalytic ineffeciency- under threshold....or some **** like that i cant remebr the exact wording.

Kronn 98SH

The answer is in the original post.

__________________
1988 Prelude S - victim of an 80mph backflip
1991 Prelude Si - sold
1989 Accord DX - impounded
1998 Prelude SH - totaled on 3/29/12
2006 CRF150F - sold
2006 YZ250F - sold

Lboni002

I got the same code a while back. I replaced both o2 sensors but still got the same code. Had to get a whole new cat. Just letting you know what happened to me.

98 Prelude base (automatic)