I have a 2005 Jeep Grand Cherokee that has a P0420 code which has recently shown up. I am not sure how long the heater has not been working as I live in GA and have just begun to start using it but the heat coming out is mildly warm. Finally, after warming up, the idle for from 1000 RPM to around 500RPM which I believe to be too low.

I know that the cat could be bad, especially given that it had a blown head gasket which was fixed earlier this year but at the same time, I think there is also a possibility the code is from something else given the other symptoms I'm experiencing.

Edit: The hose into the heater core is hot but the hose coming out is just warm. Not cold, however, so I believe coolant is flowing to the core, just not much.

2 Answers 2


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.

  • Actually, you are probably spot on about the partially clogged heater core. I used the Barr Head Gasket sealant first before replacing the head gasket altogether and there is probably residual product in there, especially given how thick the product is.
    – Ignoramus
    Dec 7, 2020 at 15:21
  • And I will check the above suggestions and report back with anything I may find.
    – Ignoramus
    Dec 7, 2020 at 15:22

Solved. Pulled all six injectors with the intention of cleaning and observing the spray pattern. All 6 were filthy, but 5 of the 6 cleaned up nicely and had a very nice spray pattern.

Just out of curiosity, I looked into the well of the one that wasn't working and saw that there appeared to be damage (from what I have no idea). Given how easy they were to get to, I replaced just the bad one with an OEM Bosch and problem solved so far. No codes thrown as of yet.

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