Should a/c SIGN remain on while I use the heat?

When I sign in to facebook my page is blankI've tried several times but my page remains blank.

could we get a model # please.
usually on left side of door.

Suggests sign of internal leak. Check dipstick for engine oil to see if there is a sign of coolant mixed with engine oil. (Generally foaming of oil instead of amber or dark solid color) (Could even show level is high due to coolant adding to oil and raising level.)
If there is, it's a very serious problem.
One quick way to try a very inexpensive fix that's worked for me is a product called Barr's Leak. (It's worked twice for me on two cars.) Get the expensive "copper" version (looks like liquid copper) and add to your coolant system following manufacturer's instructions. Top off coolant and then drive car for a couple of full engine temperature sessions.
If coolant then no longer goes down, change engine oil draining out old coolant that may have contaminated it and refill with new filter.
Keep an eye on level after that to see if coolant level remains stable.
If coolant still goes down, a engine over haul or new engine may be needed.

yes . use a code reader and reset the light in the CPU.

Hmm. Does it go out? or remains on.

If remains on, engine is telling you there could be a problem.

You will need to replace the timer. It has some sticky contacts. Here is the timer to order.

There may be a thermostat problem, but your symtoms indicate low coolant level. When coolant is too low, it will not circulate, so you get no heat in car, and the coolant remaining in engine block overheats because no cool fluid coming in.
You need to fill the system up properly then look for leaks.
With cold engine, open radiator cap. Fill with antifreeze directly in radiator. Start engine, top up rad if level drops. Put heat control on Hot. Fan on low, heat on high. Gently rev engine and top up rad as needed. IF as engine heats up, it starts kicking fuild violently out of radiator, then the thermostat is bad. if not, then recap radiator, top up overflow bottle and you're done. Look under car for any sign of leaks. Steady dripping indicates a leak, then address that.

Straight from the manual: (As you are facing the switch, the light level adjustment is on the upper left. The time selection is on the lower left.

1. With power restored and wallplate removed, use your finger or a small screwdriver to adjust the light sensitivity and time settings on the device as follows:
Light Level Adjustment:• Turn the control towards the "+" sign. Lights will turn ON in lighter conditions.• Turn the control towards the "-" sign. Lights will turn ON in less lighting conditions.
Time Selection:• Adjust the time selector to the desired length of time the lights are to remain ON. Lights will remain ON from15 seconds to 15 minutes after the room is vacated.
• Turn the control towards the "+" sign. Lights will remain ON up to 15 minutes.
• Turn the control towards the "-" sign. Lights will remain ON up to 15 seconds.
2. Test that the light level and time selection are as desired. If not, repeat adjustments until satisfied.

The manual is here:
http://www.leviton.com/OA_HTML/ibeCCtpItmDspRte.jsp?item=8018§ion=17219&minisite=10022

There are a few possibillites: a bearing is starting to go on the quill assembly or idler pulley; the PTO on the engine is heating up and quitting to work until it cools off; or you may have a switch going bad. I would look at this first.
HTH
Good Day

almost a classic sign of a bad gas valve. pilot is lit so gas is available. FAN COMES ON AND 24 VOLTS TO gas valve so there is a call for heat . gas valve doesn't open so it looks like its bad.