(2005 Jeep Grand Cherokee) P0420 Code, Heater not getting hot, and low idle after warming up

I just fixed a low idle problem on my own car. It was due to a very tiny vacuum leak past the PCV valve, around the rubber grommet that fits into the valve cover. While researching the issue on the Internet, I learned that such a leak can trigger a PO420 code. Be sure to check that the oil dipstick fits snugly, because if its o-ring is worn or missing, it too will upset the PCV vacuum; same goes for the crankcase oil filler cap. Another possibility is that the PCV valve itself is stuck, dirty or broken, allowing too much air to enter the intake. In my case, the PCV valve was the wrong part number and had to be replaced! The heavy, rubber PCV tubing that connects to the intake manifold was old and brittle, and so I sprayed it with B'laster PB and installed a hose clamp on the PCV valve, just to be sure it wouldn't leak.

The poor heat in your Jeep is probably due to a partially clogged heater core. You can try draining the coolant, then remove (or cut) the inlet and outlet hoses from the core near the firewall, and back flushing with a garden hose. If buildup is stubborn, soak the core with this flush for 20 minutes and rise clean with fresh water.