Invacare Pronto M71 stopped working. I changed the batteries and now its blinking 1 red and 3 green. I checked the manual and it said something about the left motor parking brake.

M61 flash codes, nothing for M6

https://www.manualslib.com/manual/78239/Invacare-Wheelchair-Pronto-M61.html?page=27#manual

http://www.ezmobilitybattery.com/media/pdf/Pronto%20M6%20Manual.pdf

https://www.google.com/search?q=invacare+pronto+m6+LED+Code+of+2+green+then+2+Red

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Mine is an Invacare M51 Pronto Surestep with an onboard Winsunny battery charger. My charging lights are solid red and green/ yellow. Depending on if it is charging or not. What kind of charger do you have?
I have to replace my charger because it is not working. My green light is staying green while charging even though my joystick is showing my batteries are depleting and I just got new batteries. sigh.

OK, how exactly are the 'blinking'? Are the lights simply flashing with a short pause between each sequence or are they 'chasing' each other left to right repeatedly.
The style it is flashing is a code system to give the repairer an idea as to what the problem is. If they are just blinking, count how many times it flashes after each pause. This will give a clue as to what could be wrong.
Use the comment button below and I'll see what other help I can give.

It means the battery voltage has dropped below the operating range of the chair electronics. Either need new batteries or maybe a charger and new batteries. There is always a chance of loose battery harness connections.

Throw away

Two red lights blinking means the electronics on the chair think the batteries are low. Test the voltage on the holes marked 1 and 3. Hole 1 is positive and hole 3 is negative. Your meter must be set on DC volts and the meter leads must be in Common and Volts. If your meter has to have the voltage range set, put it on 200vdc. The red lights will blink if the voltage is approximately 22.5vdc or less. The green lights turn on about 24.5vdc or higher.

You are getting a motor error code. Check motor wiring and connection to the main control box. Motor may have gotten unplugged when changing the batteries.

The 3 lights are blinking a diagnostic code. One code for batteries that are to weak is a 2 blink. The 3 lights blink together between 1 and 7 times then they all stop for a second then repeat the same amount again and again. Post the code here if you don't have a manual to look it up.

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

sounds like bad batteries