How can I fix a MacBook Air that won't charge?

Try disconnecting the battery. Reconnect the power adapter & power on the laptop. Then reconnect the battery while the laptop is still powered on. See if the battery begins to charge. Make sure to wait until the battery charges to at least 10% (more is better), otherwise you risk permanently damaging the battery. Just make sure to keep any metallic & conductive items & surfaces from touching the exposed internal circuitry while the bottom cover is removed during this procedure.

I have gotten some batteries to start charging using this procedure.

With a MBAir, there is also a possibility the I/O Cable connection on the MLB or I/O Board is loose. Also check the I/O Cable connectors & the matching connectors on the Logic Board (MLB) & I/O Board to see if any corrosion is on the inside or on the external solder joints. You may also want to look for signs of corrosion elsewhere on the exposed Logic Board, especially along the back edge of the Logic Board and also the Trackpad connector & cable on the Trackpad. If you see any signs of liquid residue anywhere, then it is possible there could be more liquid residue or even corrosion on the other hidden side of the Logic Board that cannot be seen until the MLB is removed.

You should also try the suggestions in the following Apple article for troubleshooting charging issues:

If your MagSafe cable or power adapter isn't working - Apple Support

Also make sure to unplug the power adapter from the electrical outlet for a minute in order to reset the power adapter's internal circuitry. And try an SMC Reset.

You should be able to get the Magsafe LED to light even with the battery disconnected. If this does not happen, then the battery is irrelevant in the troubleshooting process. This older laptop should power on without the battery connected.

CharlesL426 wrote:

I disconnected the battery and there are no differences. Could the I/O board be broken or malfunctioning in some way?

Certainly as well as the I/O Flex Cable. The main power cable is a heavier cable that runs around the fan beneath the I/O Cable. The I/O Cable is what actually what is needed for charging to work. I have seen the Magsafe port on the I/O Board bent if the laptop fell onto the side while the Magsafe charging cable is connected. Most times it will be noticeable upon close examination even without removing the I/O Board. Also look for any metal shavings, shards or even staples that may have gotten stuck the the magnet of the Mafsafe port. If the charging cable cannot be fully seated, then it won't work at all (there is very little clearance). Also wiggle the Magsafe cable on the port since many times a connection isn't always made the first or second time.

Do you still have the original Apple battery & would it still have a charge on it? If so, you can try connecting it to see if it will power on the laptop with & without the I/O Board connections (if the OWC battery still has a charge, then you can try it as well).

You may need to get another I/O Board & I/O Flex Cable which should not be too expensive. However, I would look closely for signs of liquid residue & corrosion on the items I mentioned previously before spending any money. It is also possible the Logic Board has failed. I've seen multiple MBAir Logic Boards have power related issues....even ones without any known liquid damage.

And you have to decide how much time & money its worth to investigate & satisfy your curiosity. If you have a local Apple repair shop, I would have them take a quick look at it. They may have known good I/O Boards & Cables for testing purposes, perhaps even a used charged battery. That is all that is needed to narrow down the problem.

CharlesL426 wrote:

The battery went flat when all of this started. It was charging fine, battery goes flat, no power and no charge indicator. Could the battery have broken something?

Yes. It could be that the replacement battery is bad, or could be that the Mac power- or charging-related hardware is bad, or something was damaged during the repair, or some other and unrelated failure.

You can usually ask Apple to look at a failed Mac, as part of getting a repair estimate.

Or in this case, check with OWC, and see what they suggest.

If your Mac battery won’t charge - Apple Support

You can probably get it repaired by a third-party repair provider, but a failed MacBook Air over a decade old isn’t usually worth an investment.