Adrian, I've been trying to diagnose similar issues as I've been adding in my dew heater code. I haven't solved this but my thoughts are that it could be:

1. Hardware problem - I'm not that neat with my soldering etc and the problems sometimes seem intermittent so I wonder whether it's just something particularly on the i2c bus. I've been gradually redoing my setup, but not sure whether this is the problem.

2. JsonDocument issue - I've been trying to understand JsonDocument to understand whether enough space is being allocated because of string usage and whether we're hitting the memory limit of StaticJsonDocument as more sensors are added. It's a confusing library, but I think the answer here is it's okay.

3. Code problems - I've been through the code in detail, whilst there are some issues in the new update like the missing preprocessor lines you identified, and some missing allocation of space in JsonDocuments, everything looks fine and practically I see similar problems with older versions of the code. So I don't think it's anything that's been recently added.

4. Memory limits of the microcontroller - I notice you're using an Uno same as me. My dew heater code is including math.h as does the Davis library, and I've wondered whether that means there's just not enough left with an Uno when it comes round to allocating memory for the Json strings.

It's been very difficult to narrow it down as it's not particularly repeatable and removing code in various places seems to fix the issue. My guess is it's probably memory, but I'm not an experienced C programmer. It's good to know though that it's not just me

-Mark

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