We have a pronto m51 and the amber light is on after we unplug it from the wall, and it won't move at all.

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Service Indicator Light Diagnostics - Invacare Pronto M51 ...

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Invacare Pronto M51 Base Manual Online: Service Indicator Light Diagnostics. ... Invacare power wheelchair owner's operator and maintenance manual.

The AMBER service indicator will light when an error or fault occurs. This is a yellow wrench shape. Refer to Diagnostic Codes on page 33 for a listing of the flash codes and what they indicate:
https://www.phc-online.com/v/vspfiles/V4_Backup/docs/MK5-NX_Electronics_Manual.pdf
https://www.phc-online.com/v/vspfiles/V4_Backup/docs/MK5-NX_Electronics_Manual.pdf

3 blinks is a right motor fault on a Pronto M51. Check that the motor plug has not come unplugged from the main control box or that any of the wires have been broken, cut or pulled out of the right motor.

Please download this manual and go to page 26:

http://search.aol.com/aol/search?&s_qt=sb&q=pronto+m51+error+code&tb_oid=07-01-2013&s_it=amonetizetest1-ie&tb_mrud=07-01-2013&tb_uuid=20130107163427450

Battery is shorted or may have cristlized replace battery. This will solve your problems.

Eduardo After using your chair and the battery level lights are down with yellow or red lights showing just plug the chair into the wall socket and let er rip. Now really after using your chair during the day and you are ready for bed just plug in the charger and let it charge over night. In the morning unplug the charger and you should be set for the day. If you don't use the chair everyday always put it up with a full charge and about once a week plug it in and let it charge overnight. Never leave the chair plugged in all the time and never leave it for more that 24 hours. The charger has two lights on it one red and one yellow or green that are hard to see because they are under the rear cover. When the chair is fully charged the yellow light turns green and you can unplug the charger and you should have a full charge. Hope this helps you

Have the batteries load tested. It sounds like the batteries need to be replaced.

the batteries were dead

How many times is it blinking?