How to Use This Guide
Answer each question based on what you observe in the DIAG log (5GS NAS OTA 0xB821, RRC 5G NR OTA) or the device's RF Monitor. The tree narrows from symptom to specific 5GMM cause code or RRC failure in 4 steps or fewer. Each outcome links to the relevant 3GPP specification or field guide.
5G Registration Failure: Interactive Diagnosis
A 5G SA registration failure can originate at the radio layer, at the NAS layer, or at the subscription level. The same symptom — device shows 4G instead of 5G — can have six distinct root causes. This decision tree identifies the layer and the root cause in under 5 steps.
Start: Does the device detect a 5G NR cell?
Check the RF Monitor (NR frequency scan) or the RRC log. Is an SS-RSRP value reported for any 5G NR cell at this location?
Frequently Asked Questions
The first step is to determine whether the failure is at the RRC layer (the UE cannot establish a 5G NR radio connection) or at the NAS layer (the radio connection is established but the AMF rejects the registration). An RRC failure shows in the RRC log as a failed RRC Setup procedure — no RRC Setup Complete is sent. A NAS failure shows in the NAS log as a Registration Reject with a 5GMM cause code. These two failure types require completely different diagnostic paths.
In field deployments, cause #43 (N1 mode not allowed) and cause #62 (no network slices available) are the most common. Cause #43 indicates the USIM is not provisioned for 5GS services in the UDM — common during 5G SA rollouts with mixed 4G/5G SIM populations. Cause #62 indicates the 5G core has no available slice for this subscriber — either the subscription is missing the S-NSSAI or the slice is locally unavailable. Cause #11 (PLMN not allowed) is the most common in cross-border or roaming scenarios.
A 5G SA coverage failure means the UE does not detect any 5G NR cell — it cannot initiate RRC Setup because there is no suitable cell. This is visible in the RF Monitor as no 5G NR frequency detected (SS-RSRP = null or < -125 dBm). A 5G SA registration failure means the UE detects a 5G NR cell, establishes an RRC connection, but the AMF rejects the Registration Request with a 5GMM cause code. The NAS log shows a Registration Reject with a non-zero cause code, while the RF Monitor shows valid SS-RSRP/SS-SINR values from the 5G NR cell.
After Registration Reject with cause #43 (N1 mode not allowed), per 3GPP TS 24.501 §5.5.1.2.5: the UE sets 5GS update status to 5U3 ROAMING NOT ALLOWED, considers N1 mode as not allowed for 3GPP access in the current PLMN, and enters state 5GMM-DEREGISTERED. The UE then attempts registration on LTE (EPS NAS, TS 24.301) if LTE coverage is available. A successful LTE Attach after cause #43 confirms the issue is 5GS subscription-specific, not device or PLMN-related.