> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hologram.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Modem & SIM annotated diagnostic test

> Run a standard set of AT commands to capture modem, SIM, registration, and APN state for troubleshooting.

## Run modem and SIM annotated AT diagnostic tests

Use a serial terminal to connect to your modem and run each command. Record the outputs for support.

<Note>
  **Note:** Some scans (e.g., network search) can take up to 3 minutes to complete.
</Note>

| Command                  | Function                                           | Sample response                           |
| ------------------------ | -------------------------------------------------- | ----------------------------------------- |
| AT                       | Attention; verifies command path                   | OK                                        |
| ATI                      | Module number                                      | SARA-U201-03B-00                          |
| AT+CGMI                  | Manufacturer identity                              | u-blox                                    |
| AT+CGMM                  | Manufacturer model identity                        | SARA-U201                                 |
| AT+CPIN?                 | SIM PIN state (Hologram SIMs do not require a PIN) | +CPIN: READY                              |
| AT+CCID                  | SIM ICCID (profile identifier)                     | +CCID: 891234...                          |
| AT+CRSM=176,28539,0,0,12 | Read FPLMN (forbidden networks)                    | +CRSM: 144,0,"FFFFFFFFFFFFFFFFFFFFFFFF"   |
| AT+CFUN?                 | Module functionality                               | +CFUN: 1,0                                |
| AT+CSQ                   | Signal quality                                     | +CSQ: 22,4                                |
| AT+CREG?                 | GSM/SMS registration                               | +CREG: 0,5                                |
| AT+CGREG?                | 3G registration                                    | +CGREG: 0,5                               |
| AT+CEREG?                | LTE/EPS registration                               | +CEREG: 2,5,"MI9S","25SS404",8            |
| AT+CGDCONT?              | PDP context parameters (APN)                       | +CGDCONT: 1,"IP","hologram","0.0.0.0",0,0 |
| AT+CGACT?                | PDP context activation                             | +CGACT: 1,1                               |
| AT+COPS?                 | Operator selection status                          | +COPS: 1,2,"310260",2                     |
| AT+COPS=?                | Networks in reach (scan)                           | +COPS: (1,"AT\&T","AT\&T","310410",2)     |

<Note>
  **Note:** Prefer capturing raw terminal text output instead of screenshots.
</Note>

## Next steps

* If `AT+CSQ` is very low, relocate the device or improve the antenna.
* If not registered (`+CREG/+CGREG/+CEREG`), verify SIM activation and supported RAT/bands.
* Ensure APN is set to `hologram` and the active PDP context matches the APN line.

## Related guides

<Card title="Automated modem diagnostics using Python" href="/guides/troubleshooting/automated-modem-diagnostics-using-python" horizontal />

<Card title="Introduction to AT commands" href="/guides/troubleshooting/introduction-to-at-commands" horizontal />