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Have been getting a P0420 for about 18 months. For the longest time, I just reset it. Had a mechanic tell me "CAT's bad" and put in a downstream sensor collar - but didn't change the O2 sensor.

I changed upstream and downstream sensors and still occasionally get the code.

Using Torque Pro and a ODB reader, I have observed upstream and downstream CAT temps.

Normal "easy" driving I am seeing an upstream temp of ~1100 and downstream of 1250. Highway speed (70mph) with cruise on will hit a pretty steady state of 1200 and 1400-1450.

Getting a temp rise so the cat is doing its job. But those temp gains seem pretty high. I would think that means either the engine is running rich all the time, I've got a cylinder that has a weak spark or a bad exhaust valve. (though I still easily get 38+mpg on extended hwy cruise and 31-33 with mixed driving). I don't have access to a pressure gauge I can stick in the O2 ports to check for any obstructed airflow.

Question 1 - anyone have "normal" upstream/downstream CAT temp numbers that I could compare to? (I think I'm using the standard CAT PIDs in TorquePro so I don't think I have a formula error.)

Question 2 - any suggestions on any other ODB data I could look at to help me troubleshoot before I try to find a reputable mechanic that won't fleece me...

Car has 120K (140K if you count 20K of four-down towing that doesn't show on the clock) and is on its 3rd clutch (all paid for by Ford) Didn't get anything out of Ford on the class action. Current clutch is starting to show signs of slip - but the warranty on a Ford part that you don't pay for is only 1 year and I was over the 100K when this latest clutch started slipping. It will need a clutch soon. Overall I've been happy but starting to wonder if it will be a money pit. If I can solve this P420 error, I may spend the money on a clutch. If not, I think I will drive it until the slip gets to bad and unload it.
 
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