Your 2003 Jeep Cherokee is showing a code P0420?

Answer 1

Provided that you have no other codes, it usually means that your catalytic converter has gone bad. Before replacing the cat, verify that you have no exhaust leaks and that the oxygen sensors are working normally. Clear the codes, then drive a steady 60 MPH for a several minutes without moving the throttle/gas pedal much. If the cat is bad, the code should come back.

Chrysler trucks tend to go through cats faster than most other vehicles, so this really isn't at all unusual. Luckily, they're comparatively cheap to replace.

To prevent recurrence of this problem:

Replace upstream O2 sensor, it's cheap insurance.

Fix any misfires.

If you haven't had a tune up recently, get one. By recently I mean 30k miles for regular or single platinum spark plugs, 60k for double platinum, 100k for iridium. Note that single platinum plugs may not provide any advantage over regular resistor spark plugs for vehicles with 1 coil per two cylinders. Single platinum is only good for distributor or COP type ignition systems.

After every oil change, park on level ground and wait 20 minutes, then check the oil level. If it's overfilled, get the oil change redone properly.

Avoid parking for prolonged periods of time facing sharply uphill or downhill. Leaning sideways or level is OK.

Avoid short trips of less than 15 minutes.

Avoid doing more than 30 minutes of low speed (under 45MPH) driving per 5 minutes of highway (50 MPH) driving. The low speed driving does not apply if it involves significant towing or a lot of hill climbing.

Answer 2

Trouble code P0420 means:Catalyst system efficiency below threshold