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Worked perfectly fine until I reconnected everything in my project. I now only get white led and red and green halflit. When releasing reset the green flashes. No recovery procedures seem to work (stop the sketch, reset).

If it is doing all that blinking, then IMHO it is not dead. I think your user sketch hangs.

By recovery procedure, you mean this?

https://z-uno.z-wave.me/getting-started ... Z-UnoHangs

"To go into Rescue mode hold down the Service Button and then hold down the Reset button, wait for a second, release the Reset button then release the Service Button. User code will not be executed until reboot (it is not erased from Z-Uno)."

You should be able to write a new sketch in that mode...

Yes rescue is what I've been using a lot since the zuno is asleep - low power mode and R12 removed.
The LEDs are not blinking, they are solid (green and red low intensity though).
When I release the reset button the green flashes at release. That's all it seems to do.
And it seems the zuno is on backorder (Belgium) so might take some time to get a new one delivered.

Ah. What did you do before it got into this state? Overvoltage? Apply more than 3.3 V to the pins? Or interrupt flashing while trying to update a sketch or the bootloader? If it is overvoltage then your chances are slim. If it is a problem with the flash image then you might have a tiny chance, There's the theoretical possibility of flashing a series 500 chip with a CP2102 serial converter. I have used it to fix a Z-Wave.me UZB1 and turn a aeotec z-stick into a zniffer.

Try rescue mode one more time, but choose "write bootloader" instead of uploading a sketch.

Thanks it just happened when I reconnected the battery.
I'm afraid it might have been a voltage spike somehow. Not sure I understand though, worked fine before and can't find anything wrong with the circuit.

Seems the voltage was indeed a little over 3.3, but not higher than 3.42.
The larger components I checked are rated for 3.6. Could one component have failed first and be replaceable?

The most probable is the ESD (spike) or short overvoltage on the Z-Wave chip. Check if you see 3V when powered by 5V. Are all chips ok or some a heating at this moment. If the voltage on 3V pin will be lower - either the Z-Wave chip is dead or power supply chip is dead (less probable). Apply 3V - check the same - is something heating? What is the current?

Faint LEDs usually mean dead Z-Wave chip or damaged firmware

Thanks for getting back. D5 seems to be dead, other diodes test correctly.
Note that I unsoldered R12 for low power long ago. Should 5V power (via usb) still work then? It doesn't but I guess if D5 is dead that explains.
5V pin measures 1.25V when powered from USB.
I already needed to power via 3V for any sketch upload before the current situation.
Current drawn from 3V power is 15mA. Nothing seems overheating.

See

https://z-uno.z-wave.me/technical/power-supply/

If you remove R12 then you effectively remove all regulators to the MCU, this is what reduces the standby current. Indeed, you have to power via the 3V or BAT pins (top of the board near USB connector).

From the top of my head, 15 mA seems "normal" for a busy CPU, so it is not in "sleep" mode... Or it is broken. It is not doing TX because that would take more current. Depends on what your last sketch was ("sleeping" versus "always on I mean")

Do you have an oscilloscope or logic analyser? Having some idea of which signals get where might give us a clue. For example, when trying to flash, do you see activity on D+ and D_ of USB port?

I don't know what killed it but can I make a recommendation? Always add a 330 ohm or even 1K ohm resistor in series with *any* input or output while experimenting. What can kill a modern chip is called "latch up" which happens when you inject current into a pin. IIRC the data sheet says a max of 10 mA is allowed. With those resistors, accidentally touching 5 or 12 V with a wire won't cause excessive current on any pin...

Thanks a lot for this very complete feedback Peter!
No oscilloscope or logic analyser (yet) in my lab. The sketch was a sleeping one. The USB port is 'connecting' but not usable - windows gives an error - I cannot start any upload.

I'll take your recommendation on the resistors. I have indeed connected everything including 2 DC step-ups without any protection.
I ordered a new zuno... Expensive learning - I mostly play with esp8266 which are another price range but the signal needs to traverse concrete/driveway so slight chance of catching a WiFi signal.
FYI the project is to measure the rain water tank fill level.