[a.graham52]

ok first off... if your SES light is on and the P0128 code is current then your fans will come on all the time (limp-home-mode). pretty much means the ecm is not seeing the temp come up high enough and sets the code. "typicaly" its the the thermostat that is to blame. they are cheap, throw one at it and bleed the coolant system. you could possibly have a coolant temp sensor issue... BUT if your temp gauge does work then your probably ok there.

P0420 cat efficiency... pretty much means your cat is not working right and if you looked at your O2 sensor voltages between sensor 1 (o2 sensor that effects fuel trims) and sensor 2(sensor behind the cat that verify operations) then you should see sensor one voltages go up and down rapidly while sensor 2 voltage stays pretty smooth and lazy. when the p0420 code sets you will find both sensors close to the same in voltages (because its seeing the same fuel and air mixture before AND after the cat) hence low cat inefficiency. you could throw a down stream o2 sensor at is because they are cheaper then a cat... however i belive your looking at a new cat.