Have a 1996 LT4 6 speed with my code reader reading 0420 or supposedly "Catalyst System Efficiency Below threshold" (bank 1).

OK I give up anyone have any idea what this code is telling me, any input is appreciated.

 

Your in luck, i'm at home and with my book. The book shows a code 0420 is: Catylist system efficiency below threshold (bank one).
Clogged cat? Good luck

 

i have a a 96 lt4 with the flowmaster force II catback with the same trouble code. the way i understand it, im new to the c4 world, the catback flows so well that it makes the rear o2 sensors think the converters are not working properly. i was told that o2 eliminators or a custom tune were needed to turn off the ses light. my car runs great so it seems possible. just my 2 cents. hopefully someone with more experience will adress this issue.good luck.

 

I hate to say it, but that sounds like crap to me.

The stock mufflers flow pretty damned good.

 

Cat efficiency is evaluated by the (OBDII) ECM. It looks at the pre & post O2 sensors (frequency). The front sensor goes high and low very fast when warmed up at high idle. The post cat sensor increases slowly & drops at flash off. If the rear O2 sensor looks too much like the front sensor, the cat efficiency code is set. A leak in the exhaust behind the cat can cause higher O2 reading in the post cat sensor. A lazy front sensor can cause the frequencies to be too similar. A graphing meter makes diagnosing this easier.

 

Wow!, Thank you to all for your input. I do love this forum. Could not live without when I had my 89 vette.

JBB "and friends" the car has 75K on her. I bought the special addition a year ago, after I got her home she had an unusual sound on the passenger side under the exhaust manifold, could be an exhaust leak, it was with the vehicle when I first got her, but was not as noticable as it is today. When you first start her, drive a little, it totally goes away. Is not all that noticable, but it is there when she starts cold. Tried to find the source from above and underneath, but have been unsuccessful. It is not the normal exhaust leak sound I am familiar with almost like a compressor or piston slap sound, but is not piston slap.

She has after market exhaust put on her to sound a little better and breath a little better, but I purchased her with those and the code light came on about a month after I bought the car some 10 months ago. The car has run perfect until just recently.

The car just hauls thru the gears, but if you shift from 1st to 3rd in traffic and if you push to hard she will buck like a bronc something she started doing recently. You shift down, let or her go and she just jumps right out from under you, not a miss or wemper, only when you lug her a little.

I cleared the 0420 code. I heard a bad MAF sensor can trip a 0420?????????

Any additional input is appreciated.