Hi everyone,

I'm building a simple monitoring system using Arduino Uno, DS18B20 temperature sensor, DHT11 humidity sensor, and a SIM800L GSM module. The system works great for sending automatic alarm SMS when temperature drops below a threshold but I can't get it to respond to an incoming STATUS request.

What works:

\\- SIM800L initializes fine (AT → OK)

\\- Automatic alarm SMS sends successfully when temperature threshold is triggered

\\- Arduino sleeps between cycles using watchdog timer

What doesn't work:

\\- When I send an SMS with the text "STATUS" to the SIM card number, the Arduino never replies

\\- No response at all, even after waiting several minutes

Setup:

\\- Arduino Uno

\\- SIM800L EVB (no RI pin available on this board)

\\- DS18B20 on D2, DHT11 on D3

\\- SIM800L TX → D10, SIM800L RX → D11

\\- Powered from 12V battery through LM2596 buck converter stepped down to 5.0V for SIM800L

\\- Yettel prepaid SIM (2G network confirmed available)

How it's supposed to work:

\\- Arduino wakes up every minute via watchdog timer

\\- Checks inbox using AT+CMGL=ALL

\\- If "STATUS" is found in any message, replies with current temperature and humidity

\\- Deletes all messages after reading (AT+CMGD=1,4)

What I've tried:

\\- AT+CMGL=4 instead of AT+CMGL="ALL" (firmware compatibility)

\\- Flushing the serial buffer before reading inbox

\\- Increasing delays after AT+CMGL to 3000ms

\\- Sending STATUS without quotes, all caps

I'm attaching the full code below. The alarm SMS part works so I know the SIM800L is registered on the network and can send messages. The issue seems to be specifically with reading incoming messages.

Any ideas what could be wrong? Is there something with how SIM800L handles incoming SMS storage that I might be missing?

Thanks!

Arduino Uno R3 WeatherStation GSM problem