2003 4.0 ck engine (P420)

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I have a 2003 Rubicon that I bought used in January. After about a week of driving it, I got a ck engine light. Code reader shows Bank one downstream O2 sensor. My question is: When I grab the exhaust right by the oil pan, it seems really loose. It appears the flange gaskets are not tight or they have deteriorated. Since the upstream O2 sensor is in the manifold before the gaskets, is it possible if I don’t have a good seal or an air leak, that could cause it to throw a code? My son has a 2001 4.0 and his exhaust is really tight. Bolts look very rusty, so it should be fun to get them to move. My 2003 only has 75k miles. Trying not t just throw parts at it.

I can clear the code, but the light comes back on between 20 to 100 or so miles.

Thoughts?

Salty Soul Jeep Shirt

Article source: http://www.wranglerforum.com/f210/2003-4-0-ck-engine-p420-2000650.html