Hi Samantha! Glad you found it useful! I try my best :)

I'm not familiar with the sim800 models although I've seen that it is what I should have used as its replacing the older sim900. Are you also posting to thinger.io or just a HTTP post request in general?

The 400 return code just means "bad HTTP request", so the request is rejected. The message basically says that "something is wrong with your request". This can come down to anything really, what you are essentially doing is building the HTTP POST request with all attention commands (AT commands) before sending it with"HTTPACTION=1". So any of the previous AT commands can be wrong in one way or another. The things I struggled with that you can check:

• Wrong/bad URL, e.g. using https and not http, wrong address, syntax error.
• Missing quoation mark around text, small things like this.
• I struggled for a long time with the byte count in "AT+HTTPDATA=<byte count>,<input delay>". This byte count has to be exact for your string which you send, otherwise it won't work. Took me quite som time to figure that one out.
• If using thinger.io, make sure the authorisation key is correct.
• the data fields in the "content" section needs to be in exact order and syntax otherwise you will get problems, e.g. { field1 : field1_value , field2 : field2_value } etc.
• Thinger.io bucket must also be set to "From Write Call" to enable HTTP write requests

If any one of these things are wrong, you might get that return code (or similar).

If none of that works, I think you are better off posting your code and serial output in the forum section, that way it's easier to see the code and what might be wrong.. it always helps to have some context