Overview
AT commands help you diagnose connectivity, SIM, registration, signal, and SMS issues. This reference includes standard 3GPP commands and vendor variations with example outputs and actions.
Note: Responses vary by vendor and firmware. Use vendor AT manuals in parallel.
1. AT - Basic connectivity check
| Response | Meaning | Next steps |
|---|
| OK | Modem responsive | — |
| ERROR | Not responding to basic cmd | Check serial, baud, power |
| No response | Host not connected | Verify hardware |
2. Identify device: ATI / +CGMI / +CGMM / +CGMR / +CGSN / +GSN
ATI
AT+CGMI
AT+CGMM
AT+CGMR
AT+CGSN
AT+GSN
ATI
Quectel
BG96
Revision: BG96MAR02A07M1G
OK
3. +CPIN? - SIM status
| Response | Meaning | Next steps |
|---|
| +CPIN: READY | SIM detected and unlocked | — |
| +CPIN: SIM PIN | PIN required | Enter with AT+CPIN=“xxxx” |
| +CPIN: SIM PUK | SIM blocked | Contact SIM provider |
| +CPIN: NOT INSERTED | SIM not detected | Check tray/orientation |
| +CPIN: SIM FAILURE | SIM unreadable | Try another SIM |
| +CPIN: PH-SIM PIN | SIM network lock | Unlock with carrier code |
4. +CCID - ICCID retrieval
+CCID: 891234XXXXXXXXXXXXXX
OK
5. +CREG? / +CGREG? / +CEREG? - Registration status
AT+CREG?
AT+CGREG?
AT+CEREG?
| Stat | Meaning | Common causes | Next steps |
|---|
| 0 | Not registered | Modem off, antenna issue | AT+CFUN=1, check +CPIN? |
| 1 | Registered (home) | Normal | — |
| 2 | Searching | Scanning | Wait / relocate |
| 3 | Registration denied | SIM blocked or wrong RAT | Check SIM/APN |
| 4 | Unknown | Transient | Retry |
| 5 | Registered (roaming) | Normal for global SIMs | — |
| 8 | Emergency only | Weak coverage | Move |
6. +COPS - Operator selection
AT+COPS?
AT+COPS=?
AT+COPS=mode,format,oper,AcT
- Modes: 0 Auto, 1 Manual, 2 Deregister, 4 Manual/auto fallback
- Formats: 0 Long name, 1 Short name, 2 Numeric MCCMNC
- AcT: 0 GSM, 2 UTRAN, 7 LTE, 8 LTE-M, 9 NB-IoT
+COPS: 0,0,"T-Mobile",8
+COPS: 1,2,"310260",7
7. +CSQ - Signal strength
| RSSI | dBm | Meaning | Action |
|---|
| 0 | ≤ -113 | No signal | Check antenna |
| 1-9 | -111 to -93 | Very weak | Relocate |
| 10-14 | -91 to -81 | Weak | Expect drops |
| 15-19 | -79 to -69 | Fair | May drop |
| 20-24 | -67 to -57 | Good | — |
| 25-30 | -55 to -45 | Strong | — |
| 31 | ≥ -51 | Excellent | — |
| 99 | Unknown | Not camped | Check registration |
8. +CESQ - Extended signal quality
- RSRP (dBm) = -140 + rsrp
- RSRQ (dB) = -19.5 + (rsrq × 0.5)
9. +CGATT / +CGACT - Attach and activate PDP
AT+CGATT?
AT+CGATT=1
AT+CGACT?
AT+CGACT=1,1
| Response | Meaning | Next steps |
|---|
| +CGATT: 1 | Attached | — |
| +CGATT: 0 | Not attached | Check SIM, signal, registration |
10. +CGDCONT - Define PDP context
AT+CGDCONT=<cid>,"<PDP_type>","<APN>"
AT+CGDCONT=1,"IP","hologram"
11. +CNMI - New SMS indication
AT+CNMI=<mode>,<mt>,<bm>,<ds>,<bfr>
| Mode | Meaning |
|---|
| 0 | PDU (raw hex) |
| 1 | Text (human-readable) |
13. +CSCA - SMSC address
AT+CSCA?
AT+CSCA="<SMSC_number>",<type>
14. +CMGS - Send SMS
AT+CMGF=1
AT+CMGS="+1234567890"
>Hello World<CTRL+Z>
+CMGS: 45
OK
15. +CME ERROR - Equipment errors
+CME ERROR: 10 // SIM not inserted
+CME ERROR: 30 // No network service
16. +CMS ERROR - SMS errors
+CMS ERROR: 1 // Unassigned number
+CMS ERROR: 27 // Invalid SMSC
17. +CFUN - Functionality level
AT+CFUN?
AT+CFUN=<fun>[,<rst>]
18. +CGEREP - Packet domain event reporting
19. +CGPADDR - Show PDP address
20. +CGCONTRDP - Read PDP context parameters
21. +COPS=? - Network scan
+COPS: (1,"T-Mobile","TMO","310260",8),
(1,"AT&T","ATT","310410",7)
22. +CSCON - RRC connection status
23. +CEDRXS / +CPSMS - Power saving modes
AT+CEDRXS=<mode>,<AcT_type>,<eDRX_value>
AT+CPSMS=<mode>[,<Requested_PeriodicTAU>,<Requested_Active_Time>]
24. 3GPP reject cause codes
| Code | Meaning | Action |
|---|
| 2 | IMSI unknown | SIM not active |
| 3 | Illegal MS | Contact provider |
| 7 | GPRS not allowed | SIM plan issue |
| 15 | No suitable cells | Move/check bands |
| 33 | Service option unsupported | APN issue |
| 40 | No EPS bearer | LTE network problem |
Appendix - Vendor-specific commands
Quectel
- AT+QNWINFO - Network & RAT info
- AT+QENG=“servingcell” - Serving cell
- AT+QCSQ - Extended signal
- AT+QCFG - RAT/band selection
SIMCom
- AT+CSERVINFO - Service info
- AT+CNETSCAN - Scan with metrics
- AT+CSQEXT - Extended CSQ
u-blox
- AT+URAT - RAT selection
- AT+UBANDMASK - Band mask
- AT+UCGED - Cell info
