No heat

2003 nissan maxima, 105000 miles. No heat, but air cond. Ok. Changed plugs couple months ago. Replaced two ignition coils & all plugs to rid misfire issues. Seems ok with changes. Still getting code 430 intermittently. Looking at getting $ to replace exhaust system from converter
back in near future. Just discovered no heat issue last few days. If trun up temp control **** to 90 degrees, have full heat. Anything below that it's luke warm. Coolent levels checked and appear full. Wondering if one hose removed while changing plugs, that had few drops of coolent come out, allowed air to get in system and create air bubble, Or, could it be a thermostat? Wouldn't think the 430 code would have
anything to do with heating, but could it? Temp guage is running in normal positon. No overheating. Comments appreciated.