94 tracer check engine light code 172

i have a 94 tracer and the check engine light comes on after the engine temp. gets to the normal line. and sometimes it goes out while driving. so i recently bought a code reader and code 172 comes out saying HEGO sensor circuit indicates lean. (Rear Side). it looks to me the previous owner may have changed the o2 sensor at least once. it is a 4 pin. i am not sure, but do they come with a 2 pin design from the factory? i have researched this and it can be a number of things. i was just wondering if anyone had a similar problem and what they did to fix it. my car has 130k miles and is a 5 speed.

not related to this problem, but my temperature just barely gets past the normal line in the summer and has a hard time getting to it in winter. i replaced the thermostat, thinking it was stuck open. this wouldn't concern me but what happens is if the temp is at the 'N' on normal and i flip the heat on full blast it comes out real nice and hot, but after about a minute or 2 it slowly cools down to just luke warm. and then my windows fog up cause there isnt any fresh air option on the heater. the heater core was recently replced about a year ago cause it leaked anitfreeze. any help is appreciated!!!!