207,000 miles

i have a p0128 and p0420

deleted this and it returned the 0420 code i understand i am looking to buy a quality catalytic converter and it is not the o2 sensor, is that correct? or do i stand corrected?
i changed the spark plugs for her and topped off her coolant and the 0128 code was deleted and never came back on..
0128 is temperature low which i was going to do a flush and use sealant as she has oil residue in her coolant indicative of seeping at the head gasket and she cant afford a head job so..

any suggestions as to which sealant is known to work best for the price need to stay under 50 she just had a baby with multiple defects and at 10 days old had heart surgery .
i wouldn't do this if i thought it wouldn't help it isn't bubbling or heavy oil in coolant so i honestly believe the sealant will work for her and she only drives local and needs the car to last 2 years to get out of it and into something better.. also plan on putting in a failsafe thermostat after flushing and sealing.. any other suggestions?? tips or tricks here ,, thanks in advance

 

P0128 is a thermostat code. Replace the thermostat and do a coolant flush.

P0420 could be related to p0128, so fix the thermostat first.

 

Thank you Aussiek2000 for that response. I was not looking forward to having tell her that she may need a catalytic converter and the high end one at that as another forum had suggested. since this was my intentions to replace the T stat with a flush and sealant for the head issue I am more relieved that it is not yet another added issue. now if someone can answer my concerns with the harmonic balanacer i will be able to get to the rest of the issues.. this one is foremost my concern before the others as it is a waste of time to do anything if this one is not resolved ..