Honeywell TH9421C1004 has some keys locked

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On Honeywell thermostats you have to program the thermostat on the setup mode to have the auto to be on the display. The setup instructions will walk up through it. If you do not have them, you can down load them to you computer.

Make sure there is 24 Volts between the R & Common wires connected to the thermostat subbase.

check your float switch. On your air hander you have 3/4 white pvc coming out of bottom of unit, where the water drains out. If you have a float and your drain line is plugged it will make your tstat go blank. vacuum the drain line where it comes out of the ground outside.

You will lose all your data if you wanna set it to factory setting ,have its back up before you loose it.

Here is how to restore at Factory Setting..
1: Restart the computer, click Start, click the arrow next to the Lock button, and then click Restart. 2: As restarts, press the F8 key until the 'Advanced Boot Options' ,menu appears on the screen.It should come up before Window logo appears. 3. Press the (Down Arrow) to select Repair Your Computer on the Advanced Boot Options menu, and then press Enter. 4. Specify the language settings that you want, and then click Next. 5. Log in as a user who has administrative credentials, and then click OK. 6. Click Dell Factory Image Restore. 7. In the Dell Factory Image Restore window, click Next. 8. Click to select the check box for Yes, reformat hard drive and restore system software to factory condition. 9. Click Next. The computer is restored to the default factory configuration. 10. When the restore operation is completed, click Finish to restart the computer.

Instructions

1. 1 Unlock a partially locked screen by pressing any locked key, which will bring up the "Screen Locked" message. Next press an unlocked key, which will likely be the temperature up and down arrow keys. This will unlock the screen.
   
2. 2 Press the "System" key from the home screen. Five blank touch keys, flanked by the "Done" and "Cancel" keys, will appear. Hold down the middle key for about five seconds. The Advanced Settings menu should come up.
   
3. 3 Change the User Setup Number, found in the bottom left of the screen, by pressing the middle arrow keys up or down. Set the number to "0670," the universal unlock code.
   
4. 4 Turn to the arrow keys on the right side of the screen. Press the keys up or down to change your options. Selecting the number "0" will unlock the screen, while choosing "1" will partially lock it. You can fully lock your screen again by selecting.
   

Read more: How to Unlock the Screen on a Honeywell Thermostat ' eHow.com http://www.ehow.com/how_7667347_unlock-screen-honeywell-thermostat.html#ixzz19ktDWSq8

Access the ISU (installer setup) menu by pressing the system key from the main screen then press and hold the two keys on either side of the center for approximately five seconds. Hit next until you find 0670 and then change the value to 4. http://www.forwardthinking.honeywell.com/related_links/thermostats/commercial_pro/install/62_0221.pdf On a side note, this information was insanely hard to find.

Go to Advance Functions Page 23 of your manual.
You have to turn Option 0160 to "on" or "4"

1) Press System
2)Press and Hold blank center key until the display changes.
3) Change settings as required (Cursor to option 0160, then hit other cursor to change from 0 to 4)
4)Press Done to exit and save changes

Solution #2 as above worked (thank you very much!) to get to the programming mode, but on mine keeping Function #13 on the default setting of "0" maintained the AIR.  Toggling the Temp button to "1" changed the system to conventional on/off at the programmed times.