Samsung ML-2510 Red light on & won't print.

This may be a help. I had a similar problem with an ML-2850 which fresh out of the box refused to print, not even a test page, green light followed by red and the smart panel showed that the printer thought that there was no toner in it.
After struggling with it for an hour I hit upon the following.
Turn off the printer.
Take out the toner cartridge.
In the hole where the toner came from there's a gear on the left hand side which is sort of diagonal mesh to match the underside of the toner cartridge. Rock it back and forth a bit and then stick the cartridge back in.
My guess is, if you're unlucky when you install a new cartridge you could have a situation where the gear is in exactly the wrong position, tooth to tooth with the underside of the toner so this gear doesn't mesh properly, the cartridge doesn't seat properly, which then shows the printer as having no toner in it. Waggling the gear just changes the position slightly and then voila. A bit like rocking a car back and forth in gear to free a stuck starter motor.

Here's a suggestion and some analysis

The CHEAP toner cartridge samsung provides with the printer is ONLY rated for 1,000 pages.

The toner cartridge you BUY for around $85.00 is rated for 3,000 pages

If power switch is ON, hold down the red button for about 4 seconds for a test page, showing
total unit page count, toner page count, as well as TONER cartridge capacity.

I started having printing problems at 800 pages and shook cartridge for more usage (about another 50 pages)

also:
Double sided printing:

After first sheet prints, you MUST turn over page manually, and then the light will flash
hit the (you guessed it, RED button) to feed the reverse side.

This is actually a good system, since it requires that 2nd feed, and gives you time to do it.

ALSO: when printing business cards, make SURE you use the "Cardstock" option, otherwise the paper doesnt feed correctly

10 times better then anything since my LaserJet III
except for cartridge prices

Had problem where red light went on and would not print. Was persistent for days. I can not say how it started but seemed to arise spontaneously. Would take out toner cartridge shake it, replace it, still persisted with red light and would not print. Tried rotating the toner roller with the diagnolly hashed sprocket on the bottom, as suggested here, that did nothing. Had tried removing plug and reinserting, that did not thing. Removed and reinstalled all removable trays, closed and reopened doors on device several times, through many restarts, all returned to red light non-working state. The printer is used infrequently, and toner should be near full, so was thinking some defect existed perhaps in the toner cartridge. Then decided to open one last time, and noticed that the toner cartridge while appearing to be inserted and seated properly was not and so pulled the cartridge out and carefully reinserted watching right side in particular, and saw that it fully inserted properly particularly on the right. Close it, and now it reset itself and the green light came on and prints properly. So after much frustration it seems there was nothing wrong with it, but that the toner cartridge can be inserted and not seated properly though it may seem so.

I had a similar problem... The cartridge has 2 possible positions when inserted and one is wrong... it seems that cartridge has to pushed all the way in and then caught by the closing printer front cover to stay in.

Z.

Have you tried to change the fuse?

O.K. we all have the same problem. Red light is on, no jam and won't print. What is wrong with it and how do you fix it?

You have gasoline spread it all in the unit and burned...

Put cartridge back in a couple times. No luck.
Cartridge shaken couple times. No more luck.
Turned the thing down.
Slammed the printer the desk (not too wildly).
Put paper back it.
Turned it up.
Now works.

Had the same problem, paper in tray was not fully inserted