I had an issue last week where I had no sound coming from my ND’s speakers, no matter which audio source was selected. I could select radio stations and move between sources without problem… there was just no sound.
I had a search round here but couldn’t see any similar posts on this one, so thought I’d write this up in case anyone else sees this in future.
I did the usual system reset a couple of times, no change. I left the battery negative terminal disconnected for a few hours, no change. I also checked the relevant fuses for any issue - all were fine.
Next I scanned the car for fault codes - I used a Windows laptop, diagnostic cable and Versatuner to read the codes. This came up with a B1238-01 fault, which came back as:
Speaker circuit malfunction
“The connectivity master unit (CMU) detected a short to ground or short to power supply in the speaker circuit”
I cleared the code, reset again, but there was no change and upon rescanning the error code was logged again. So I moved onto further troubleshooting steps: Manual pages - MX5 Manual | Mazda MX5 (Mk4) 2015+ - my particular issue was with all speakers, so see Step 26. This moved me onto inspecting the TAU (tuner & amp unit) itself. I used this guide for more detail on location and subsequent de-installation and removal: https://www.hexorcism.com/16ND/sm356303/
Everything looked OK with the unit itself, and wiring to it. And there was no sign of water ingress which had crossed my mind - all nice and dry. So I went ahead and removed the TAU so I could confirm the part number and order a replacement. I obtained one from an ND at a breaker’s yard via a reputable salvage company on eBay. I did notice quite a bit of price disparity, so shop around 
The replacement unit arrived a couple of days later and I installed it. Access is a bit tricky given it’s up to the right of the driver’s footwell. But about 20 mins later it was in place and I gave it a test. Re-connected the negative battery terminal, allowed the CMU to fire back up… and there was sound again. Hooray! I did a few more ignition power cycles just to be sure - audio came back as expected each time. So I went ahead and secured the new TAU and tidied everything up.
Job done. Hope this is useful in case anyone else comes along and does a search with a similar issue, or with that same error code.
Cheers,
Steve
