First key bit of information is that HP / Agilent are absolutely spectacular companies for providing service manuals, product manuals, and datasheets well beyond the retail life of their instruments.  You can still pick up documentation directly from Agilent here.

In digging through the datasheet you can see that a FAIL 023 means we’re having a problem with the dc offset, so I printed off the relevant service information (groups K,J) and prepared for the worst.  After powering the machine on at my desk I was again met with the FAIL 023 when requesting a sine wave, and a new error was a FAIL021 when requesting a square wave.  I then out of chance cycled through the other 3 settings (triangle, etc) and returned to the square wave and sine waves to discover that the errors had went away!  It’s likely that I had a stuck/hung up relay and switching away from square and sine waves cycled something into place again.  We’ll see how long it lasts but hopefully I’ve just picked up a nice piece of equipment.

Additionally while I was reassembling the covers, turning the instrument from side to side resulted in a nice pinball sound, I swung it back in forth until I was greeted with 3 washers on my desk, I can’t imagine that these were helping with things but they’re out now.  Looks like they came off the main power transformer which someone had helpfully duct-taped into place at some point.