The temperature gauge on my Pathfinder is always on "cold"

While it doesn't seem likely, if you've changed, or had the thermostat changed.... I'd say you got a faulty thermostat. I am curious as to what your temperature gauge says the temperature of the engine and coolant is.. If your gauge is showing hot, I can think of two more possibilities... the water pump isn't pumping right... and, if the water pump isn't working right, the problem can be a cracked head or block, which can cause an airlock in the coolant system. Check the coolant or radiator cap.

I would suspect the thermostat first.

REPLACE THERMOSTAT IF TOP COOLANT COOLANT HOSE GOING TO RADIATOR IS HOT TO TOUCH THERMOSTAT OPEN. TO BLEED COOLANT SYSTEM FIRST YOU NEED TO ADD COOLANT TO THE COOLANT OVER FLOW JUG UNTIL COOLANT LEVEL STOP DROPPING AND STAY AT FULL COLD MARK.WAIT 3 MINUTES IF COOLANT DROPS ADD MORE COOLANT UNTIL COOLANT STAY AT FULL COLD MARK.WITH RADIATOR OVER CAP OFF CRANK ENGINE FOR MINUTE WATCH COOLANT LEVEL AT OVERFLOW JUG.IF COOLANT DROPS ADD MORE COOLANT IN THE OVERFLOW JUG.WHEN COOLANT LEVEL STOP DROPPING IN OVERFLOW JUG.SCREW BACK ON THE RADIATOR CAP TO OVERFLOW JUG.WHILE ENGINE IDLING WATCH TEMPERATURE GAUGE WHEN TEMPERATURE START CLIMBING TURN OFF ENGINE WAIT A WHILE KEEP FROM GETTING SCALDED.WHEN COOLANT COOL DOWN SOME. PLACE A LARGE RAG OVER THE COOLANT OVERFLOW JUG OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED, THEN REMOVE CAP ADD MORE COOLANT IN THE OVERFLOW JUG TO THE FULL COLD MARK.THEN REPEAT THE PROCESS SCREW BACK ON THE COOLANT OVERFLOW RADIATOR PRESSURE CAP.THEN START ENGINE WATCH TEMPERATURE GAUGE WATCH COOLANT TEMPERATURE GAUGE WHEN COOLANT TEMPERATURE START RISING TURN OFF ENGINE AGAIN WAIT UNTIL ENGINE COOL DOWN ENOUGH TO KEEP FROM GETTING SCALDED TAKE PLACE LARGE RAG OVER RADIATOR CAP WHICH IS THE COOLANT OVERFLOW JUG CAP OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED ADD COOLANT UNTIL AT FULL COLD MARK. WHEN YOUR COOLANT LEVEL STOP DROPPING.AND TOP RADIATOR HOSE VERY HOT THERMOSTAT OPEN AND AIR IS OUT THE COOLANT SYSTEM. MAKE SURE YOUR COOLANT OVERFLOW JUG HAS COOLANT IN IT,WHEN ENGINE COOL DOWN CHECK COOLANT OVERFLOW JUG KEEP COOLANT LEVEL AT FULL COLD MARK.BECAUSE IF OVERFLOW JUG IS EMPTY THATS A SIGN RADIATOR COOLANT LEVEL LOW.WHEN OVERFLOW JUG EMPTY IT WILL ALLOW AIR GET IN THE COOLANT SYSTEM AND CAUSE CAR TO OVERHEAT.WHEN REPLACING THERMOSTAT ALSO REPLACE THE RADIATOR CAP ON THE COOLANT OVERFLOW JUG.

Sounds like bad thermostat, very common issue. Replace thermostat and add new coolant and you'll be rolling again.

Replace the thermostat. the temperture should be at the 190 to 200 mark

When your engine thermostat has failed or is sticking open, the engine fluid isn't able to reach a temperature that lets your heater core get hot enough to produce worthwhile heat to blow into the passenger's compartment. It is also not good for your engine to run below design temperature.
If you have a dashboard temp gauge, if your thermostat is stuck open, that gauge will probably stay in the cold area.
I would think that that is the problem, and I would have the thermostat changed (not a very difficult or expensive job).
Hope this helps you.

Since your blower is working and it is a heat issue,thats where we'll start,First thing is have you checked the coolant level in the radiator when engine is cold,make sure it is filled to top,next verify you have coolant in reserve ,Start your van and get it up to normal operation temperature,If your heat gauge on the dash works correctly it should be 190 degrees,Now lift the hood and locate the two heater hoses going into the heater assembly at the fire wall,Both hoses should be hot to hold on to,if both are hot and no heat inside your problem will be the blend air door in heater assembly.
if neither hose is that hot you need to replace the thermostat in the engine to get it up to 195 degrees.
if one hose is hot and one is luke warm then the heater core is blocked and to blame. hope this helps y

The sensor for the fans is in the rad at the bottom. T-stat may be bad or the rad may be clogged. Cooling system needs attention.

As long as your gauge is still moving up, there's no real problem. You have heat coming from the heater. It's only a problem if it's getting too hot. What's going on is that you don't let the vehicle run long enough, and it's so cold outside that the engine stays cold. It raises in temp when your stopped because there's no airflow through the radiator. The thermostat just heats up the engine block, and won't change the reading on the gauge. When this goes bad your car will overheat in just a few miles.

Sounds like either your thermostat, which is the least expensive and easiest thing to change, or your water pump, or clogged radiator, which is a whole new world! The reason it wont blow hot, is because you are not getting any water to the heater core. Hope this helps.