3.7L Jeep Liberty misfire

[masanders]

My sister just bought a 2009 Jeep Liberty w/3.7L V6.
It has a P0305 Cylinder 5 misfire and a P0420 cat code. The plug and coil pack were changed at the dealership which did not fix the problem. I hear a hiss but can’t find a vacuum leak. It’ll get all new plugs this Thursday thanks to the dealer.

Any suggestions?

 

[Dirtman]

Check compression on #5 before wasting any time or money on it. They are notorious for spitting out valve seats. Once that happens your buying a new engine. Chunks of valve seat smashing into the cat would also explain the p0420...

 

[sheep herder]

F I A T = Fix It Again Tony

 

[RonD]

+1 on the compression test

I would think dealer would do that first before changing a spark plug or coil

Specific cylinder misfire is one of the big 3
compression
spark
fuel injector


P0420 means downstream O2 is either worn out, or Cat has a problem
How many miles on the vehicle?
O2s tend to last 10-12 years or 150k miles, downstream O2s can last a bit longer, 300k miles but years are years, so.....

 

[Dirtman]

F I A T = Fix It Again Tony

Fiat didn't buy Chyrsler until 2014...

 

[sheep herder]

Fiat didn't buy Chyrsler until 2014...

Jeep = just empty every pocket