Bongo Fury

After doing some work on resoldering some old connections, tidying various cables and switches, and replacing some dash bulbs. I now have a hold light flashing. I wired up an led and went through the Engine, ATX and ABS codes on the data port and only have errors on the ATX, which mirrors on the dash.

The codes are 21, 11 and 16 which I can't find info on. Any ideas please?

V6 2001

Try searching on the web for the full 4-digit version "mazda bongo error code P0021"
OR
its possible your codes are the 4-digit versions as it was around 2001/2002 that they changed over in which case you need to read the codes in that format and not the Ten's and Units format.

As a four digit code it seems to be coming out as 1601. A web search leads me to this description:

P1601 Powertrain Control Module Communication Line To TCM Error

How on earth do I check this? It could possibly be a loose connection or something?

Try this to reset the fault codes

Remove the wire from the negative terminals on all batteries (including leisure battery if installed) for at least 20 seconds.

Click on the brake pedal and release.

Re-connect negative battery terminal.

Start engine, and run for 3 minutes at 2,000 rpm idle.

Check all fault codes have cleared.

Thank you for the instructions. The error codes have cleared so I will see how things go from now.