Copier wont work alarm light is on says paper tray is open.

depends on what alarm is on for ? if its on for say ? out of paper or out of ink then no its not going to print , even if it really has paper and ink , but it thinks it is out it still wont try , paper out switches in the tray do get bent/broken and some mfg wont allow u to use non mfg ink/toner it just gives a fault

If the paper out tray is closed or not fully opened, then open it fully.
If that doesn't fix it, then the printer may believe that paper is jammed. You've already checked for jammed paper, so reload the paper supply and press the BLACK or COLOR button.
If that doesn't work, then disconnect the power for a few minutes and reconnect.
If that still doesn't work, there is probably a bad paper sensor or controller board.
Is your printer still under warranty?

You have a mismatch paper size. Document is not same as paper you have in copier. Try printing 8 1/2 by 11 document if doesn't print check printer preferences and adjust settings. Check copier paper trays make sure don't have special settings on them

Check the Paper Tray Sensor where you insert the tray.

Try powering it on and off. Also check to make sure there is no paper jams and that the paper in the tray is snug up against the rails that hold it in place.

Set paper in ASF, and
press the OK button.&yor printer open cling

Pull the tray out and make sure the back guide is pushed against the paper. The back guide tends to move easily on these models

Not sure what model you have but it always helps to open all doors ans trays and check carefully if there is any paper stuck, if so remove and shut all doors. If you still have the message and are sure there is no paper stuck switch off then on again,

Do you mean the "Out of paper" alarm? Check that all paper trays have paper. The by-pass tray should be prperly set to receiv "Legal" size paper and have that size paper in that tray.

THE PROB WITH THE PAPER TRY WOULD HAVETO BE FIXED FIRST...YOUR ONLY OTHER OPTION IS TO REMOVE THE 2nd 500 SHEET TRAY COMPLETLY AND ONLY WORK WITH THE 150 SHEET TRAY.