Auto Problem - Check Engine light

[FrankSaucedo]

1998 Honda Civic LX
8 Months ago I had a cracked exhaust manifold. I replaced it with one from a salvage yard.

3 weeks ago the check engine light came on. The code is P0420 Catalyst efficency below threshold - bank 1. Before I go out and purchase another exhaust manifold (no, i wont get it from a salvage yard again) is there any possibility that the sensor may be bad??? How do i check the sensor?

Thanks in Advance.

 

[Fiveohhh]

http://64.233.167.104/search?q=cache:fcp0IXIP-BQJ:forums.vwvortex.com/zerothread%3Fid%3D1366095+%22Catalyst+efficiency+below+threshold%22+p0420&hl=en

thats for impreza, but sounds like the same thing

 

[rayray2]

Could be the cat.