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Catalyst Efficiency Below Threshold--

Old 05-22-2008, 09:38 PM
JimBlake's Avatar
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Join Date: May 2006
Location: Wisconsin
Posts: 17,516
Default RE: Catalyst Efficiency Below Threshold--

ORIGINAL: finch13

I'm going to have to argue that one. My 1999 has had the P0420 Cat Below Threshold code since 2001, that's 7 years with the code and has not suffered a loss in mileage. In fact, the other week I got 34 MPG.
Yeah, me too. Losing thePRIMARY (front) O2 sensor causes it to go into open-loop. The rear O2 sensor doesn't close the control loop at all. It's just monitoring the cat.

Considering the secondary O2 sensor is located downstream of most of the catalyst, it's not evenmeasuring the proper thing to close the fuel-control loop.
Old 12-22-2016, 11:40 AM
Join Date: Dec 2013
Posts: 133

Reviving an old tread. I have the same message on my 2000 Accord 4 cylinder. It says below threshold on bank 1. Is that the front sensor or rear?
Old 12-22-2016, 06:08 PM
TexasHonda's Avatar
Super Moderator : And A Texan
Join Date: Feb 2007
Location: Katy, TX
Posts: 9,651

If you got dtc P0420, the problem is not likely the secondary O2 sensor but the catalytic converter.

If you have a scanner you can check the secondary (behind converter) voltage trace for faulty converter operation (frequent 1-2 hz, voltage swings from 0.1 -0.9 volts. If that pattern is present the converter has failed.

Or you can simply replace the secondary O2 sensor and see what happens,

good luck
Old 12-23-2016, 10:13 AM
Join Date: Dec 2013
Posts: 133

Thanks Tex! I do have a scanner, I'll check for the voltage fluctuation. I noticed by looking at other sites that the front O2 sensor fluctuates but the rear is supposed to remain steady.

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