Your Dodge truck is probably suffering from a lean fuel mixture. I'll bet
the CO levels are low. It may appear that the CO being low is a good thing,
but technically it's not. See, the CAT needs a little fuel to work properly.
A lean fuel mixture not only causes the engine to run hot (NOx is created
when combustion chamber temperature go above 2500f; lean fuel does not help
cool the engine down)) but it also doesn't let the catalytic converter start
to work. Start with making sure the fuel feedback system is working
properly. This includes making sure the oxygen sensor and all emissions
sensors which tell the computer how much fuel to send the engine are working
properly.
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If I'm wrong and the problem is not air/fuel related, check the EGR system,
obviously including the EGR valve. Your Dodge must have an exhaust
restriction through the EGR system; whether is be carbon build up or
defective EGR components, i.e.. EGR valve, EGR control solenoid, vacuum
hoses ect.
