My 02 ssei failed emmisions because of two codes 0420 and 011something one was about the coolant being to low all the time and the other was about my cat. which i replaced like 6 mmonths ago. any help would be awesome

0420 - Catalyst System Efficiency Low

Not sure what the 011 is......

P0110 Intake Air Temperature (IAT) Sensor Circuit
P0111 Intake Air Temperature (IAT) Sensor Performance
P0112 Intake Air Temperature (IAT) Sensor Circuit Low Voltage
P0113 Intake Air Temperature (IAT) Sensor Circuit High Voltage
P0114 Intake Air Temperature Circuit Intermittent
P0115 Engine Coolant Temperature (ECT) Sensor Circuit
P0116 Engine Coolant Temperature (ECT) Sensor Performance
P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage
P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage
P0119 Engine Coolant Temperature Circuit Intermittent

If it'* this one.....P0128 Coolant Thermostat Below Thermostat Regulating Temperature.......it'* common and nothing to worry about. You either need a new stat or you have been driving far in really cold weather and allowing the engine to warm up fully before shutting it down again.

You should probably replace your Cat with a better one. :?

i have a magna flow cat on it now. he said that it could be an oil leak in the exhaust valve or something i do have a oil leak in the valve cover

It is possible that you could be losing fluid from the Lower Intake manifold.

Oil leaking from the valve cover will not cause a problem other than a mess.

You need to look for internal loses.
Check your plugs. Have you replaced them?

Start with a good tune up. Plugs, wires and throttle body cleaning if dirty.
What is your mileage?