Yes the codes are in the database for Xantech's PA1235 (AUX = code 420).

So reading this....
Either you did a horrible job and the customer really didn't want to use you again. Or the customer is really stupid and/or price shopper who cares about nothing more than price.

If the latter, I would seriously consider telling the customer I'm not available. "I have loyal customers I need to attend to".
Either that or I would let them know up front, that it was going to be expensive to fix everything - and I wouldn't touch the job unless I was fixing everything. Note: get money upfront.

There are soooo many red flags with your post.