We have 3 Intermatic TB121C timers. 2 of them stopped working yesterday; the switch will no longer turn the lights on or off. We were not using the timer function at this time,just the switch. The display is still on. Will replacing the batteries help. The units are plugged in.

1) Trippers can wear down. Transformer can go bad if model has 24Volt output. Relay can go bad.
http://waterheatertimer.org/pdf/How-to-Set-and-Troubleshoot-Intermatic-sprinkler-timer.pdf
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#sprinkler

2) Add a comment Feb 2013 with model number, and include if this is new timer or older timer that worked before and stopped working.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Is this new timer installation, or older timer that stopped functioning?

1) Copy following link to help identify timer, download manual:
http://waterheatertimer.org/Intermatic-timers-and-manuals.html#E1020

2) If this is new installation, then check wiring for single pole installation. This timer will not work for 3-way switching.
http://waterheatertimer.org/How-to-wire-timers.html

3) If timer worked before, and now clock motor has stopped working:
Turn power off, use compressed air to blow out timer, then see if you can spray highly flammable vapor from WD40 can into the motor.gears.

4) There are replacement trippers for this timer, but no replacement clock motors.
Buy another single-pole in-wall timer.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Replace timer.
Copy following link to identify timer
http://waterheatertimer.org/Intermatic-timers-and-manuals.html#KM

If analog timer starts hanging, then motor is probably getting weak.
Use air pressure and blow out timer while rotating timer dial.
Turn power off and spay bit of highly flammable vapor from WD40 can while rotating timer dial.
Timers generally have 1 year limited warranty.
Replace timer.
http://waterheatertimer.org/How-to-wire-timers.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Model number of timer is not known.
Is this new installation? Or installation that worked before and recently stopped working?
If the manual override lever on timer does not turn Load on-off, then 1) wiring is wrong or 2) timer has gone bad or 3) pump has a short.
I suspect a short since you say the pump 'tries to stop running.'
Copy following links for troubleshoot walk-thru, and correct wiring:
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timers.html
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

You probably have T104 timer.
Clock rotates so power to timer motor is good. This means 240Volt breaker is good.
Use override lever and see if Load (light, fan, motor) turns on and off. If override does not turn Load on-off, then bypass timer and see if load works. Move wires from terminal 2 to terminal 1, and terminals 4 to terminal 3.
Did you order correct trippers?
Copy following links for Intermatic troubleshooting and Intermatic parts list.
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timers.html
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#T100

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

You posted under EH40 Intermatic programmable timer.
EH40 Manual:
http://waterheatertimer.org/pdf/eh40-manual.pdf

1) EH40 uses size A battery. Replace battery.
http://waterheatertimer.org/How-to-wire-EH40-hot-water-heater-timer.html#Battery
2) Reset the timer: Press and hold reset button for several seconds
3) Contact Intermatic because they are aware of specific problems with specific timers
http://www.intermatic.com/customer%20service.aspx
4) Other folk have reported EH40 and EH10 last about 3 years.
EH40 has 1 year limited warranty.
I test timers: 7 years is reasonable run for inexpensive electronic time switch.
5) For longer lasting timer, consider Next Generation: battery is replaced every 8 years
http://www.intermatic.com/products/timers/electronic%20time%20switches/next%20generation%20time%20switches.aspx

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.

That's a good old timer.
Grasslin ET724 in-wall timer is no longer available.
http://www.amazon.com/GRASSLIN-ET724-Grasslin-Et724-Electronic/dp/B000BHCFT6

There are several programmable timers on the market.
Intermatic EJ500 is simple to install and program. Works on battery but is for incandescent light only, not for fans.
http://www.amazon.com/Intermatic-EJ500C-Indoor-Digital-Switch/dp/B000E8VAIO

GE 15071 from Home Deport is inexpensive replacement and works with fans, lights.
Needs no battery. Uses electric circuit to operate clock.
http://waterheatertimer.org/GE-timers-and-manuals.html#15071

More timers:
http://waterheatertimer.org/Programmable.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you via e-mail while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.

1) If this timer was working fine before and stopped working, then timer is defective.

2) For warranty claims and parts:
Intermatic Inc.7777 Winn Road, Spring Grove IL. 60081
https://parts.intermatic.com/

3) If this is new timer or existing installation went bad, then check that circuit is clean.
Use circuit analyzer on outlet on same breaker.
Circuit problems can affect timer electronics.

4) Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on timer, or any do-it-yourself project.

You posted under EJ500 Intermatic battery-operated timer.
If you have different timer, add a comment.
I have tested EJ500 timer.

A) If this is new single-pole installation and lights click on-off when timer turned on, then you connected red wire where blue wire should be connected.

Manual says for single pole switch:
Timer-black-wire to Hot; Timer-blue-wire to Load (lights)
Here's how to wire timer correctly:
1) Timer-green-wire connects to bare ground wire.
2) Switch has 2 wires. Take switch wires and connect them to timer-black-wire and timer-blue-wire. Choose either one for now.
3) Put wire nut on red wire.
4) Turn on electricity, and push ON-OFF button. If lights come on, timer is wired correctly. If lights do not come on, then reverse timer-blue and timer-black wires.

B) If you are replacing 3-way switch, view wiring diagrams shown on link below, and then add a comment saying exact color of wires. To wire this timer for 3-way can require additional wire between switches. This is because EJ500 blue wire must always go to Load (light, fan, motor)
http://waterheatertimer.org/ST01C-program.html#EJ500-wire

C) If this is older timer that stopped working, check your bulbs. Then check timer battery. Manual says if timer stops working and bulbs are good, and battery good, then reset timer and reprogram. How to reset timer. Press reset button while holding down Next/ON-OFF button.
EJ500 manual has long list of programming instructions: Add a comment if you forgot parts of programming.
EJ500 manual
http://www.intermatic.com/~/media/files/intermatic/products/instructions/timers/ej500c%20-%20english.ashx

DT121 Manual
http://www.intermatic.com/~/media/files/intermatic/products/instructions/timers/dt121c%20-%20english.ashx

All Intermatic manuals
http://www.intermatic.com/customer%20service/instruction%20manuals.aspx

Intermatic does not show DT11
Intermatic shows the DT121 is no longer available

If the display shows and you can set programs, then the battery is good.
Does the manual override work? This indicates whether the timer on-off switch operates.
If manual override does not work, then I discovered that timers are no longer any good

Programming:
Set present time
Then set ON time for program 1
Set OFF time for program 1
Then set ON time for program 2
Set OFF time for program 2
Most timers will not function if there is a conflict in programming > for example Program 1 says ON at 6 and OFF at 7 and Program 2 says opposite or has overlapping times
When events are set, press SET once > this will put timer in RUN mode

If manual override works, and you set programming correctly, but timer does not carry out instructions, I guess the timer is no good.

Intermatic has 24 hour timers with 2 on-off per day
http://www.intermatic.com/products/timers/consumer%20indoor%20timers/lamp%20%20appliance%20timers.aspx
Intermatic has 7-day digital timers (all digital timers today are 7 day where you select the day that each program works)
http://www.intermatic.com/products/timers/consumer%20indoor%20timers/digital%20timers.aspx