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During a trip from Oregon to Cali and back the CEL came on.
I went to 2 parts stores when I got home and had the codes read.
(Thank you O'Reillys and AutoZone)
Both stores verified P0055. Post CAT H2O sensor. Bank 1, sensor 3
I felt like I might need a scanner of my own so purchased one and it also read P0055.
Purchased and installed a Motorcraft sensor.
Cleared short and long term ECU memory by removing neg battery cable.
Turned on headlight switch to assist in draining capacitors in system (per recommendations here in this forum and several other sites).
Waited half hour and re-connected.
Started and idled for 5 minutes and drove varying conditions for 50 miles.
CEL stayed off the whole time.
Stopped at market for food supply.
Less than 2 miles away from market CEL came on again.
Scanner says P0055 again.
Rinse and repeat two more times with same results.
80k miles on odometer seems to be too soon for a CAT to fail. ???
Any help or experience with this would be much appreciated.
 

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I found the same result for the code, with more detail here:http://engine-codes.com/p0055_ford.html

Sensor failure is ONE of the options, with poor contact at connectors & wiring failure the others.

It's NOT a sensing failure, but involves the heater circuit of the sensor (heats it up for quicker response after starting). As a result, you can figure on it NOT being a "CAT" problem.

hopefully that'll point you in a direction to help find the problem, and relieve at least one worry.
 
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