I have a pronto m51 and just put new batteries in it. After making sure they were fully charged It just has the wrench flash yellow 3 times. What's the deal?

8 flashes is a Controller Fault

https://www.manualslib.com/manual/723349/Invacare-Pronto-M50.html?page=25#manual

https://www.google.com/search?q=m51+pronto+sure+step+flash+codes

All questions need 'make model (what it is)' or it is all guesswork and clairvoyance 'Anonymous'.

A battery operated massage Chair, powered wheelchair? Give us a clue!

Google has some ideas and shows Questions here on Fixya. Blink Codes listed by make are readily available and are in your manual.

The power arm is flashing a wrench and it blinks 8 times - Fixya

https://www.fixya.com > support > t14332970-power_a...Nov 14, 2019 - Turn the chair off, wait 5 seconds, turn the chair back. If chair still does not run and your wrench is flashing you have a problem.SOLVED: A wrench light is on the indicator and the chair - Fixya
Sep 9, 2019What does the wrench blinking mean on the wheelchair and ...
Oct 5, 2016
SOLVED: What do eight wrench flashes indicate? - Fixya
Dec 6, 2019
Pronto m51 electric wheelchair yellow wrench light blinks 8
Sep 5, 2019
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On the Invacare M51 a flash code of 3 usually indicates M1 left motor fault. Check the motor connections and Anderson pins in the connector housings. The motor can be tested using a voltmeter. The motor windings should read about .4 ohms and the brake about 40 to 60 ohms. Hope this helps.

recheck your polarity on the batteries carefully, i've done it for 15 years and the factory makes it complex sometime i connect backwards initially

why did you change the batteries, did you verify that it wasn't the charger in the first place, sometime the charger will appear to charge but actually isn't. put a meter on one battery then plug in charger if voltage goes up charger is good if not look for loose connections and or bad charger.

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

With an M51, a full charge will be indicated by all three joystick indicator lights lit- red yellow green. The green far right LED is the full charge light. Also the charger has indicator lights that go from red/yellow to full green at full charge. Hope this helps

on my m51 the lights on the charger are red and green the green is flashing what is this telling me.also should i have the controller on when i plu in the charger my chair i 6 years old battery replaced 2 years ago

If it's the yellow battery light it will blink when the battery voltage is very low, too low to drive and lower than the red blink.

When you say "control box" I assume that you mean the joystick. If there is no power to the joystick be sure that the batteries are fully charged and be sure that they do not need to be replaced, also be sure that the cables were not put on wrong. If all that checks out then be sure that all the cables are tight.

If all these do not solve the problem then you need to have the joystick tested.