Sanilas427

I have a 2001 ls430 that keeps draining the coolant from the radiator and no matter what the temp gauge reads. Temp gauge runs normal ,at below halfway mark, but doesn't over heat. But once it hits the halfway mark, ( which isn't far from running normal )it overheats. So no matter how long or short my trip, my coolant is gone and I have to refill , but I do not see it leaking from anywhere. The codes being thrown are p0420, p0430 and p1350. Can someone tell me what it going on? I did just replace the knock sensors per code p1663. Please help as I cannot afford coolant any longer. Thank you to anyway that can help shed some light on this.