Replaced the expansion tank due to leaking. Low coolant message still on. Did the bleed, etc. The old sensor is open when low, and shorted when full. The new sensor works just the opposite.

Sensor in the fill tank is probably bad .
LOW COOLANT
The IPC illuminates the LOW COOLANT indicator in the message center when the IPC detects a low coolant level condition from the PCM. The IPC receives a class 2 message from the PCM indicating the coolant level.
Coolant Level Control (If Equipped)
The engine cooling system contains an engine coolant level switch to alert the driver in the event of a coolant loss. The powertrain control module (PCM) sends out a coolant loss signal over the coolant level switch signal circuit. When the engine coolant level switch reads a low coolant level in the fill tank, the switch opens. The message center receives its power from engine wiring harness junction block on the battery positive voltage circuit. Ground is provided by the ground circuits via the body wiring harness junction block and the engine wiring harness junction block. The cluster receives the class 2 message from the PCM indicating Low Coolant and displays the LOW COOLANT LEVEL message on the driver information center (DIC).

Is the fill tank full ?

Either the low sensor in the tank is stuck, you have not connected the sensor wire properly or you bought the wrong tank.

if its a xj350 check the wiring as it can go open circuit.due to corrosion.

not the pump but air in the system or a broken wire to the coolant level sensor
that sensor at the top of the radiator tank has to be completely immersed in coolant or the circuit is broken and the light comes on
same effect id the wire to that sensor is broken in the insulation

hello my8,
First confirm that the engine is actually overheating - steam, no heat to the interior etc. Sometimes a temperature sensor will fail and give a misleading 'overheat' reading on gauge. For that matter sometimes the gauge can fail and give the same misleading reading. Check that the cooling fan comes on when the engine temperature starts to rise above 'normal'. If not check (in order) fan fuse, fan relay, fan sensor, fan. Real overheating is caused by a lack of coolant AND/OR lack of coolant flow. Modern cooling systems are 'closed'. In other words there should be little or no loss of coolant. If the level is significantly low you have a leak....either external or internal. With the coolant topped up to the correct level and the radiator cap on, run the engine and look for visible leaks...hoses, valves, radiator etc. Internal leaks, eg. head gasket will usually be accompanied by white exhaust smoke (steam), hard starting, rough running and bubbles in the expansion tank. Sometimes, though NOT always, you will fiand the engine oil has a milky look and/or engine oil will be mixed in with the coolant.

Lack of coolant flow can be caused by anything in the system that causes an obstruction......in order of common occurance......thermostat stuck closed, radiator blockage, heater core blockage, water pump failure. One last point. because of design considerations many vehicles have the radiator mounted at a different level than the uppermost coolant passages in the engine thus trapping air in the coolant passages. In short the system must be bled of air in order to properly fill with coolant. Such systems will incorporate bleed screws usually in either the top/side of the radiator, the top hose, the thermostat housing and sometimes in all of these points. To bleed out the air do NOT run the engine as you fill the radiator. Instead, open all bleed screws. As you fill the radiator coolant will run out of one or more bleed points. Close this bleed point(s). Continue to fill until coolant escapes from the next bleed point then close that one. Proceed until coolant has escaped from ALL bleed points then continue to fill to the correct level. The expansion tank will have minimum and maximum level markings. Fill only to the minimum level mark with the engine cold.

hope helpful,
greetings

I WOULD CHECK BOTH RADIATOR HOSES TO MAKE SURE THEY ARE HOT. THERMOSTAT COULD BE STICKING SLIGHTLY CLOSED.CHECK COOLANT LEVEL MAKE SURE COOLANT NOT LOW.IF COOLING NOT BOILING OUT OF THE EXPANSION TANK. I WOULD HAVE CAR HOOKED UP TO A DIAGNOTIC SCANNER AND CHECK COOLANT TEMPERATURE SEE IF TEMP. OVER 230 DEGREES IF SO RADIATOR COOLING FANS NOT WORKING.COOLANT THERMO SWITCH BAD BOTH FANS SHOULD BE RUNNING AT LOW SPEED.IF ALL LOOKS GOOD YOU COULD HAVE FAULTY LOW COOLANT SENSOR IN EXPANSION TANK.MAKE SURE COOLANT IS IN THE EXPANSION TANK BECAUSE COOLANT LEVEL COULD BE LOW.ADD COOLANT IN THE EXPANSION TANK UNTIL YOU AT MAX LEVEL LINE ON THE EXPANSION TANK.DONT OVER FILL THE EXPANSION KEEP ADDING COOLANT UNTIL COOLANT LEVEL STAYS AT THE MAX LEVEL LINE ON THE EXPANSION TANK.

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.

If this ever happens again turn the internal heater to full, it helps in the cooling. When filling the engine with coolant the heater should be on full. As the engine warms squeeze the top radiator hose regularly to encourage air bubbles to shift to the expansion tank. If the coolant level looks low after the engine has been brought to full running temperature and air has been purged from the system then top up with additional coolant. Note that the signal to run the fan is sent by the coolant sensor, if the coolant is low the sensor cannot function. Check your engine error codes, sometimes if the coolant sensor cannot detect normal running temperature (perhaps due to low coolant level) the engine is unable to enter closed cycle mode and reports p0125. Closed cycle is the normal running state of an engine at normal running temperature; open cycle denotes the times when the engine is cold such as at start up or when the engine is under load such as when accelerating hard.

It could be low on coolant,or air in the cooling system,or the coolant low sensor needs cleaning.

there can be a yellow light in the cluster that looks like a small radiator or box with waves inside of it to indicate water. This light means that the coolant level sensor has detected a low coolant level. Typically the expansion tanks inside of these cars will crack from heat and age. A simple diagnosis is to check your coolant level and top it off. Do not forget to bleed it by unscrewing the bleeder screw found on the upper radiator hose while filling up the reservoir found on the right hand side of the radiator. once the coolant has been filled, tighten the bleeder screw back down but be careful not to over tighten. and finally screw on the expansion tank cap. run the vehicle to operating temperature and use a flashlight to inspect the engine bay for leaks. If there is a leak, it will need to be repaired. possible leaks are a cracked expansion tank, leaking radiator hoses, or a leaking water pump or thermostat.