Codes po420 and po 442 keep popping up on my 2005 Cadillac deville it has 90,000 miles what would be the most serious issue do you think all the most likely issue wrong with my car

SOURCE: 1997 cadillac deville concours stability reduced message

press and hold the off and red warmer button on your ac control panel / the display should then go into diagnostic mode.  all the trouble codes can be read out to help in tracing down the problem. you can also read out other info by pressing the temp up or down button when in diag. mode.   press the off button when your done.

SOURCE: Codes PO 442, 449 and 455

Check all vacuum lines from the gas tank to the evaporative charcoal cannister by the gas tank and to the purge valve located at the engine. Check the electrical connector at the vent solenoid which should be located on the charcoal cannister and purge valve solenoid by the intake manifold.

SOURCE: Cadillac Deville 2000- Engine check light code P0741

If you like go ahead and let someone sell and replace your transmission but here is my suggestion! Check around and see if you can find a minor vacuum leak! This can simply be the big round hose that goes from the fuel injection to the power brake hub! This is normally a push in connector which has about a 1/2 inch hose clamped on it! If there is any kink or the connector is not in correctly a leak will occur and the transmission will shift rough as though you will be thrown out of the vehicle! Try unplugging this connector and plug it back in hard and straight and maybe you will be lucky and fix your problem. If the vacuum is correct your transmission will shift better than you have ever experienced!

SOURCE: keep getting code po420

Before Checking a Catalytic
Converter DTC
Before you troubleshoot an OBD II vehicle that stores
DTC P0420 (67) (catalyst system efficiency below
threshold), run these quick checks:
1. Check for a leak in the exhaust system. If you find
one, repair it, clear the DTC, and test-drive the
vehicle.
• If the DTC returns, go to step 2.
2. Connect the PGM Tester, and test-drive the vehicle
while an assistant monitors the voltage signal from
the secondary oxygen sensor (HO2S S2). After the
catalyst reaches operating temperature, the HO2S S2
voltage should stay between 0.5 and 0.8 V at steady
cruising speed. During deceleration, the voltage
should be steady at 0.1 V or less.
• If the voltage readings are OK, clear the DTC.
• At cruising speed, if the voltage fluctuates or stays
below 5 V, go to step 3.
3. Measure the inlet and outlet external temperatures of
the catalytic converter with a thermometer capable
of reading up to 500°F.
• If the outlet temperature is more than 100°F hotter
than the inlet temperature, the converter is OK.
• If the outlet temperature is less than 100°F hotter
than the inlet temperature, replace the converter.

SOURCE: my 2001 cadillac deville keeps giving me dtc code system to lean

Anytime I get a too lean DTC I first change the fuel filter if it hasn't been changed in awhile. You'd be surprised how many times this has helped.

Good Luck with it.

Bob