Quick Reference
5GS NAS cause codes are one-byte values defined in 3GPP TS 24.501 §9.11.3.2 (5GMM causes, Registration Reject) and §9.11.4.2 (5GSM causes, PDU Session Establishment Reject). 5G SA introduces cause codes specific to N1 mode authorization and network slicing that have no LTE equivalent. Captured via DIAG log code 0xB821.
5GS MM Cause Codes: Field Reference
5G Standalone (SA) NAS introduces two layers of rejection: 5GMM cause codes in Registration Reject (access and subscription level) and 5GSM cause codes in PDU Session Establishment Reject (session and slicing level). Understanding which layer failed — and which node generated the cause — is the first step in any 5G SA field diagnosis.
Cause codes in Registration Reject messages. Govern: N1 mode authorization, PLMN subscription, network slice availability, and PLMN-level access restrictions. Generated by the AMF.
Cause codes in PDU Session Establishment Reject messages. Govern: DNN availability per slice, resource allocation within slice, and SSC mode support. Generated by the SMF, forwarded by the AMF.
Cause Code Quick Reference
Source: 3GPP TS 24.501 §9.11.3.2 (5GMM) and §9.11.4.2 (5GSM)
| Code | Hex | Name | Type | NAS Message | Diagnosis |
|---|---|---|---|---|---|
| #43 | 0x2B | N1 mode not allowed | 5GMM | Registration Reject | Full guide → |
| #62 | 0x3E | No network slices available | 5GMM | Registration Reject | Full guide → |
| #67 | 0x43 | Insufficient resources for specific slice and DNN | 5GSM | PDU Session Establishment Reject | Reference only |
| #72 | 0x48 | DNN not supported or not subscribed in the slice | 5GSM | PDU Session Establishment Reject | Full guide → |
| #73 | 0x49 | Insufficient resources for specific slice | 5GSM | PDU Session Establishment Reject | Reference only |
| #74 | 0x4A | Not supported SSC mode | 5GSM | PDU Session Establishment Reject | Reference only |
Per-Cause Diagnosis Guides
AMF rejects 5G NAS (N1 mode) registration because the UDM has no 5GS subscription data for this IMSI. UE enters 5GMM-DEREGISTERED and may fall back to LTE. Common during 5G SA rollout with mixed 4G/5G SIM populations.
Read full guide →None of the UE's Requested NSSAI entries are included in the Registration Accept's Allowed NSSAI. All PDU sessions are blocked. Common when per-S-NSSAI rejection causes (TS 24.501 §9.11.3.46) are aggregated: subscription mismatch or local slice unavailability.
Read full guide →SMF queries UDM and finds the requested DNN is not listed under the subscription context for the given S-NSSAI. Registration and slice are both valid — only the DNN-to-slice binding is missing. Most common in private 5G slicing deployments.
Read full guide →5G Network Slicing and Cause Code Hierarchy
5G SA cause codes reflect the layered nature of network slicing: a UE can succeed at one layer and fail at the next. Understanding the hierarchy prevents misdiagnosis.
The UE is not authorized to use 5GS NAS at all. UDM has no 5GS subscription data. No slicing check is performed — the failure is before NSSAI evaluation.
N1 mode is authorized, but none of the requested slices are available in this PLMN. NSSAI evaluation fails for all requested S-NSSAIs. Registration itself is rejected.
Registration succeeded, slice is granted (in Allowed NSSAI), but the DNN is not subscribed for that slice in the UDM. Only PDU session establishment fails; the UE remains registered.
DNN is valid for the slice, but the UPF or SMF has no capacity for a new session. A retry after congestion clears may succeed.
S-NSSAI Reference (3GPP TS 23.501)
| SST Value | Slice Type | Use Case |
|---|---|---|
| 0x01 | eMBB | Enhanced Mobile Broadband — consumer internet, video streaming |
| 0x02 | URLLC | Ultra-Reliable Low Latency — industrial automation, remote surgery |
| 0x03 | MIoT | Massive IoT — sensors, meters, low-bandwidth devices |
| 0x04 | V2X | Vehicle-to-Everything — autonomous driving, DSRC applications |
Capturing 5GS Cause Codes in the Field
Capture NR5G NAS OTA messages in real time. Registration Reject and PDU Session Establishment Reject are decoded with cause IE, Allowed NSSAI, T3346/T3396 timer, and PDU session parameters (DNN, S-NSSAI, PDU session ID).
Verify Requested NSSAI vs. Allowed NSSAI in real time. Identify which S-NSSAIs are granted, which are rejected, and which per-S-NSSAI cause code applies. Critical for private 5G acceptance testing and slice validation campaigns.
After cause #43 (N1 mode not allowed), observe the UE's LTE fallback via DIAG code 0x713A (LTE NAS EMM plain OTA). Confirm LTE attach succeeds, isolating the failure to 5GS subscription and not to the device or radio.
GPS-correlate 5GMM rejections to map 5G SA registration failure zones. Distinguish between no-SA-coverage areas (no 5G NR cells detected) and SA-registration-rejection zones (5G NR cells present but cause code returned).
Frequently Asked Questions
5GMM (5GS Mobility Management) cause codes appear in Registration Reject messages and indicate why the UE could not register with the 5GS AMF — they govern access, subscription, and PLMN restrictions. 5GSM (5GS Session Management) cause codes appear in PDU Session Establishment Reject messages and indicate why a specific PDU session (data connection) was refused by the SMF — they govern DNN availability, slice binding, and resource allocation. Both are defined in 3GPP TS 24.501.
5GS NAS messages (Registration Reject, PDU Session Establishment Reject) are captured via DIAG log code 0xB821 (NR5G NAS OTA). The decoded message includes the 5GMM cause IE (for Registration Reject) or the 5GSM cause IE (for PDU Session Establishment Reject) with the cause code value and all associated IEs (Allowed NSSAI, T3346/T3396 timer, PDU session ID, DNN, S-NSSAI).
Yes, and this is common with slice-specific session failures. A Registration Accept confirms that the UE is registered and that the requested S-NSSAIs are in the Allowed NSSAI. A subsequent PDU Session Establishment Reject (cause #72: DNN not supported in slice, or cause #67: insufficient resources for slice and DNN) means the slice-level registration succeeded but the specific DNN is not available or resources are exhausted for that slice. The UE remains registered throughout.
In 5GS NAS (TS 24.501), cause #11 carries the same semantics as in LTE (TS 24.301): the PLMN is not allowed for this subscription. The UE adds the PLMN to the forbidden PLMN list and initiates PLMN selection. The 5GS cause code numbering mostly mirrors LTE for common causes, but 5G-specific causes (slicing, N1 mode) use codes above #40 that have no LTE equivalent.