I have a brother hl-1240 that has both the yellow data and red alarm led's lit. and it just printed 10 blank pages

From the Service manual:

Toner low: The Data and Alarm LEDs blink once every five seconds.
Toner empty: The Data and Alarm LEDs blink once every second.

Alarm & Data LEDs are ON: Toner empty

If I could be of further assistance, let me know. If this helps or solves the issue, please rate it and give a testimonial for my response

Thanks, Joe

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_8b8c2cd6ce148309

your toner is empty or drivers are not installed correctly 

red alarm light is on and printer is not printing

Your toner cartridge is empty. Time to replace it.

If you look at your manual for error codes or on the brother website they will show what what the various blinky lights are telling you.

the first alarm light (at the very top) indicate the toner life ends the second light(middle) indicate the drum life ends and the third is for paper jam or print error

Because I was getting gray shading on my pages and data was still being printing, I decided to change the toner cartridge. Now the pages are clean and white and all the lights function properly. Most cartridges that are running on empty don't print any data at all or at least print the data at the top of the page and gradually gets fainter sp? on the page. Who would have thought just by changing the cartridge would solve the problem? So glad that's all it was........wasn't excited about having to buy a new printer.

Did you pull the plastic tape on the new cartridge?

Generally if both the (red) alarm LED and the (yellow) data LED are both blinking, it is an indication that the tone cartridge needs to be replaced. You should be close to out of cartridge.

check the sensors maybe stuck

what kind of alarm your printer gives