FAQ
TL;DR: One proven fix is to flash the latest correct stock ROM with Odin. “The problem is solved by uploading another software.” [Elektroda, rafallos5, post #16973267]
Why it matters: This FAQ helps Galaxy J3 (2016) owners who see “Connection problem or incorrect MMI code” when topping up via USSD and want safe, practical fixes.
Quick Facts
- Symptom: top‑up USSD fails with “Connection problem or incorrect MMI code,” while balance check works. [Elektroda, AvatarGT, post #16969192]
- Factory reset alone did not resolve the error on the affected J3. [Elektroda, AvatarGT, post #16969493]
- Internet and calls function normally despite the MMI error. [Elektroda, AvatarGT, post #16972807]
- Flashing the highest available factory firmware via Odin is the recommended remedy. [Elektroda, tino2003, post #16973448]
- Model variant matters (J320F vs J320FN); use firmware that matches your exact device. [Elektroda, beo, post #16976076]
What does “Connection problem or incorrect MMI code” mean?
It indicates a USSD/MMI session failed between your phone and the carrier. USSD codes (like *123#) use a real‑time connection for tasks such as balance checks or top‑ups. When this link breaks or the phone interprets the code incorrectly, you see that error. [Unstructured Supplementary Service Data]
Why can I check balance but can’t top up on my Galaxy J3 (2016)?
In the reported case, balance inquiries worked but top‑ups failed on the same device and SIM. That points away from network or SIM faults and toward a phone‑side software issue affecting certain USSD transactions. [Elektroda, AvatarGT, post #16969192]
Will a factory reset fix this MMI error?
Not always. A full reset did not help in the thread case. If the firmware or CSC is mismatched or altered, only flashing the correct stock software typically clears the issue. [Elektroda, AvatarGT, post #16969493]
Does swapping SIM cards help diagnose the problem?
Yes. Another SIM in the same J3 still showed the same error, suggesting the phone software, not the SIM, was at fault. Try another handset with your SIM to confirm. [Elektroda, AvatarGT, post #16969265]
Is this hardware or software?
Evidence points to software. One expert noted the error appeared after FRP removal using a combination file and was resolved by flashing different firmware. “The problem is solved by uploading another software.” [Elektroda, rafallos5, post #16973267]
How do I fix it by flashing stock ROM with Odin?
Three‑step How‑To:
- Download the latest factory firmware matching your exact J3 (2016) model.
- Flash it with Odin as full stock (AP/BL/CP/CSC), not a combo file.
- Reboot, then test USSD top‑up.
This approach was recommended by experienced members. [Elektroda, tino2003, post #16973448]
Where should I download the ROM for the J3 (2016)?
Use SamFirm to fetch the latest official package, or browse Sammobile for a wider selection. Ensure the region/CSC and model match your device. [Elektroda, rafallos5, post #16974448]
What if SamFirm flash fails or Odin stops at recovery.img?
That can happen with a mismatched or problematic package. One user reported three failed attempts ending at recovery.img and a temporary soft‑brick before recovery. Re‑verify model/CSC and use a different correct firmware build. [Elektroda, AvatarGT, post #16976948]
How do I confirm J3 model variant (J320F vs J320FN) before flashing?
Check the model ID in Settings > About phone or in Download Mode. Flashing the wrong variant’s ROM risks failure or boot issues. An experienced user emphasized choosing F vs FN correctly. [Elektroda, beo, post #16976076]
My internet and calls work. Is the issue limited to USSD?
Yes, that pattern matches the thread case. Data and voice were fine, while only USSD top‑up failed. This further implicates firmware or CSC handling of MMI/USSD, not radio hardware. [Elektroda, AvatarGT, post #16972807]
What is FRP (Factory Reset Protection), and why is it mentioned here?
FRP is an Android security feature that requires a linked Google account after certain resets. Devices modified for FRP removal using “combination files” can show atypical software behavior, like USSD errors, until proper stock firmware is restored. [Hexnode Help Center]
Is there a quick workaround so I can add credit today?
Yes. Move your SIM to another phone to perform the top‑up, then return it to the J3. This worked reliably for the affected user while the J3 issue persisted. [Elektroda, AvatarGT, post #16969192]
What if flashing the latest stock ROM doesn’t help?
Try a different official firmware build for your exact model/CSC. An expert observed the issue with one build and resolved it by switching software. Quote: “The problem is solved by uploading another software.” [Elektroda, rafallos5, post #16973267]
Could a “new‑looking” used phone still have prior service modifications?
Yes. Even boxed devices can carry prior service changes. In this thread, the seller said the phone was new, yet FRP‑related history and firmware anomalies were suspected. Treat such symptoms as signs to restore clean stock firmware. [Elektroda, AvatarGT, post #16969284]
