I replaced heat sensor on engine and fan wont come on but if i unplug sensor it runs and low coolant light comes on but its not low what is the problem this is a 1997 sunfire

A pontiac montana ? There are three relay's that control your cooling fans ! unplug what sensor ? The engine coolant temp. sensor is an input to the PCM - engine computer . When temp. reaches 213 degrees the PCM energizes the relays , depending on low or high speed . Using a scan tool to see if the PCM will turn on the fans an checking engine coolant temp.data is the right way to diagnose this . Relay Driven Dual Cooling Fan Diagnostics

It`s a 2000 buick regal ls. make sure when it gets hot eh fans are running. There is a coolant sensor just below the top radiator hose ON THE ENGINE. This sensor tells the fans when to go on when the engine gets hot and also serves as a low coolant sensor. unplug the sensor and replace it. use a open end or box wrench. if your quick you can replace it and only loose a few ounces of coolant. Hope this helps and God Bless :O)

the "sensor " near the thermostat should have 2 ,3 or 4 wires to it
if it only has one wire , that is the temp gauge sender unit and not the coolant sensor the coolant temp sensor reports to the ecm and that unit then switches on the fan relay that spins the fan
if you unplug the coolant sensor and the fan starts to run you have a serious coolant temp sensor circuit problem
take it to an accredited service center and have it sorted

Cause the sensor is unplugged,can't control it any more,will always run now.

CHECK COOLANT FAN FUSES AND RELAYS. CHECK COOLANT LEVEL.MAKE SURE ITS NOT TOO LOW BECAUSE IF COOLANT TOO LOW . BY NO COOLANT AROUND OR COOLANT SENSOR NOT SUBMERGED IN COOLANT COOLING FANS WONT TURN ON.MAKE SURE YOU HAVE 50/50 ANTIFREEZE AND WATER. CHECK FOR CLAPSE RADIATOR HOSES.CHECK RADIATOR HOSE AT WATER PUMP.IF HOSE LOOK CLAPSE REPLACE IT AND BLEED COOLANT SYSTEM.IF EVERY THING HAS BEEN DONE YOU NEED ENGINE BLOCK FLUSHED OUT.HEATER CORE ALSO COULD BE STOPPED UP.A VERY HIGH MILEAGE ENGINE WILL OVER HEAT.OLD ENGINE LOSES COMPRESSION AND HORSE POWER.PUTTING MORE LOAD ON THE OLD ENGINE WILL MAKE IT OVER HEAT.IF COOLANT LEVEL IS LOW.BLEED COOLANT SYSTEM.START THE ENGINE LET IT IDLE FEW MINUTES WATCH THE COOLANT TEMPERATURE GAUGE.WHEN TEMPERATURE START RISING.TURN OFF THE ENGINE LET IT SET FOR 20 MINUTES.TAKE A LARGE RAG PLACE OVER RADIATOR CAP ON COOLANT EXPANSION TANK SLOWLY OPEN RADIATOR CAP JUST A LITTLE TO RELEASE A LITTLE PRESSURE AT A TIME.DONT OPEN CAP RAPIDLY OR YOU WILL GET SCALDED.ONCE THE RADIATOR CAP REMOVED.IF COOLANT LOW ADD MORE COOLANT IN THE THE COOLANT EXPANSION UNTIL YOU GET TO THE COLD MARK.REPEAT THIS PROCEDURE START ENGINE LET IT RUN UNTIL TEMP GAUGE START CLIMBING A BIT DONT LET CAR RUN TO OVER HEAT JUST IDLE UNTIL TEMP JUST START TO RISE.THEN YOU TURN OFF ENGINE. WAIT 20 MINUTES.THEN USE LARGE RAG OPEN RADIATOR CAP LITTLE AT A TIME. DO THIS PROCEDURE UNTIL THE COOLANT IN THE EXPANSION TANK STOP DROPPING. THEN ALL THE AIR IS BLEED OUT THE COOLANT.WHEN COOLANT LEVEL STOP DROPPING, WHEN YOU DRIVE CAR AROUND.AND TURN OFF ENGINE.WHEN ENGINE OFF CAR SET FOR A WHILE KEEP CHECK ON THE COOLANT LEVEL IN COOLANT EXPANSION TANK. THE COOLANT LEVEL SHOULD BE AT THE COLD MARK IN THE EXPANSION TANK.MAKE SURE COOLANT IS IN THE EXPANSION TANK.IF NOT AIR WILL DRAW IN COOLANT SYSTEM. CAUSING ENGINE TO OVER HEAT.IF BLEEDING AIR OUT THE COOLANT SYSTEM DONT HELP AND COOLANT FANS DONT TURN ON YOU HAVE PCM PROBLEMS.

Yes It could be the problem due to the proper seal against the seal on the cap. If the heating system is fine and well it shouldn't be the thermostat...................But the proper flow through the engine may effect the temperature sensor for the fan.................Hope this helps.

THE ENGINE IT WONT CRANK IF OVER HEATED CHECK COOLANT LEVEL IF LOW ADD COOLANT.REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP.IF ENGINE STILL OVERHEAT AT A STANDSTILL OR IN A LONG LINE OF SLOW MOVING TRAFFIC.COOLANT FAN SHOULD COME ON.IF ENGINE OVERHEAT CHECK TO SEE IF COOLANT TURNS ON IF NO.CHECK COOLANT FAN FUSE AND RELAY.IFBOTH OKAY HOTWIRE COOLANT FAN TO SEE IF FAN RUNS IF NO REPLACE COOLANT FAN MOTOR. IF FAN RUNS PROBLEM COULD BE PCM OR THE COOLANT TEMPERATURE SENSOR FAULTY OR DAMAGE WIRE CONNECTOR TO COOLANT FAN.USE A CODE SCANNER TO LOOK FOR PCM OR ECT FAULTS.IF EVERY THING CHECK OUT OKAY LOOK AT ENGINE OIL DIP STICK.IF IT LOOKS LIGHT BROWN OR LIKE MILK SHAKE YOUR HEAD GASKET BLOWN.MAKE SURE YOU HAVE 50/50 ANTIFREEZE AND WATER TO ENGINE TO HELP RAISE BOILING POINT.WHEN OUTSIDE TEMPERATURES HOT.

Sounds like you have a faulty head gasket or maybe a cracked head.This can cause overheating and the check engine light to come on.The code for the o2 sensor is probably caused by coolant getting into the exhaust system and coating the sensor.The coolant fan may work but it cannot remove enough heat from the rad to make the engine cool off.Try some engine coolant stop leak to see if it will help.This maybe only a temperary fix.Let me know if you need more.MOE

NO, the sensor is good.

if i disconect temp sensor fans come on cool engine but then no temp gauge

That's what I'd expect after unplugging.
This is sounding like a bad coolant temp sensor. Test it or replace it before you do anything else. Read this post on how to test an engine coolant temperature sensor

Test engine coolant temperature sensor Ricks Free Auto Repair Advice