Help with Carrier 38G250HP-1

Installed this wall mounted split for a client 12 years ago and it has just started playing up. Wont cool or heat just shuts down. Indoor fan will run but as soon as comp is asked for it shuts down. Green run light is blinking 5 times, which according to Carrier tech support was a failed comp or low charge. Charge OK so replaced comp and have the same problem.

Carrier tech support is at best not very helpful. I have been trying to get info on a service manual from them, not available, or more too the point wont share.
I have spent hours looking on line to no avail.

Can anyone help with where I might find a service manual or at least a flow diagram to fault find with? I really need to repair it as it was installed during a renovation and cant be replaced, and yes its on R22!