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Ford U0121

built on or bef

DriveUnknown. The supplied verified evidence does not establish whether the vehicle is safe to drive with U0121.
ChargeUnknown. The supplied verified evidence does not establish whether the vehicle is safe to charge with U0121.
ServiceService need is listed as unknown in the supplied diagnostic record. However, qualified service evaluation is appropriate to determine whether the documented Ford TSB scenario applies.
Confidence95%

Quick answer

For a 2021 US Ford Mustang Mach-E, DTC U0121 is documented in Ford TSB 25-2071 as a code that may be stored in the gear shift module (GSM) along with certain other communication codes. The supplied evidence identifies GSM software as the possible reason in the bulletin scenario, but it does not provide a complete definition of U0121.

This page separates verified manufacturer/regulator evidence from supporting material. Unsupported causes, repairs or safety conclusions are not guessed.

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1. What does U0121 mean?

For a 2021 US Ford Mustang Mach-E, DTC U0121 is documented in Ford TSB 25-2071 as a code that may be stored in the gear shift module (GSM) along with certain other communication codes. The supplied evidence identifies GSM software as the possible reason in the bulletin scenario, but it does not provide a complete definition of U0121.

2. Technical explanation

The verified source is NHTSA Manufacturer Communication 11015681, Ford TSB 25-2071, published March 13, 2025. Its supplied excerpt states that some vehicles may have DTCs U0100:00, U0121, U0140, and/or U0155 stored in the gear shift module (GSM), and states that this may be due to software in the GSM. The excerpt also references a "System Fault" message and DTC U0103:87 stored in the secondary on-board diagnostic module (SOBDMC). The diagnostic record's canonical title and meaning are incomplete ("built on or bef"); therefore, a full code definition is not established by the verified evidence.

3. What system is affected?

Gear shift module (GSM), based on the cited Ford TSB. The secondary on-board diagnostic module (SOBDMC) is also referenced in the bulletin excerpt in connection with DTC U0103:87.

4. How serious is this code?

Unknown. The supplied verified evidence does not assign a severity level.

5. Symptoms you may notice

  • A "System Fault" message is referenced in the supplied Ford TSB excerpt.
  • The evidence does not establish that this message occurs in every case where U0121 is stored.

6. Possible causes

  • Ford TSB 25-2071 states that the listed GSM codes, including U0121, may be due to software in the gear shift module (GSM).
  • No other cause is established by the supplied verified evidence.

7. Can I still drive the vehicle?

Unknown. The supplied verified evidence does not establish whether the vehicle is safe to drive with U0121.

8. Can I charge the vehicle?

Unknown. The supplied verified evidence does not establish whether the vehicle is safe to charge with U0121.

9. What you can safely do

  • Arrange diagnosis through Ford service or another qualified service provider, particularly if a "System Fault" message is present.
  • Do not attempt high-voltage or module repair procedures based on this information.

10. Step-by-step troubleshooting

  1. Confirm which module stored U0121. The cited bulletin specifically references U0121 stored in the gear shift module (GSM).
  2. Have a qualified service provider check whether Ford TSB 25-2071 applies to the vehicle's exact build and condition.
  3. No further troubleshooting procedure is established by the supplied verified evidence.

11. When to contact service

Service need is listed as unknown in the supplied diagnostic record. However, qualified service evaluation is appropriate to determine whether the documented Ford TSB scenario applies.

12. What the service center may check

  • Retrieve diagnostic trouble codes and identify the reporting module.
  • Check for the Ford TSB 25-2071 condition involving U0121 and other listed GSM codes.
  • Evaluate GSM software applicability in accordance with Ford service information. Detailed procedures are not provided in the supplied evidence.

13. Possible repair or resolution

  • The supplied evidence indicates GSM software may be involved.
  • The exact repair, software action, and outcome are not established by the provided excerpt.

14. Affected vehicles

15. Related TSBs / service bulletins

  • Ford TSB 25-2071, NHTSA Manufacturer Communication 11015681, published March 13, 2025. The excerpt states that some vehicles may store U0121, U0100:00, U0140, and/or U0155 in the GSM due to GSM software.
  • Applicability supplied: 2021 Ford Mustang Mach-E, US. The excerpt's build-date limitation is incomplete in the supplied evidence ("built on or bef"), so the exact build range is not established.

16. Recall information

  • Not yet established by verified evidence.

17. Official manufacturer information

Official source: NHTSA Manufacturer Communication 11015681, Ford TSB 25-2071: https://static.nhtsa.gov/odi/ffdd/tsbs/TSBS_RECEIVED_2025-2026.zip#nhtsa_id=11015681

19. Frequently asked questions

What does U0121 mean on a 2021 Ford Mustang Mach-E?

The supplied diagnostic record does not provide a complete verified definition. Ford TSB 25-2071 identifies U0121 as one of several codes that may be stored in the gear shift module (GSM) in its documented condition.

Can GSM software cause U0121?

Ford TSB 25-2071 states that the listed GSM codes, including U0121, may be due to software in the GSM.

Is it safe to drive or charge?

Not yet established by verified evidence.

20. Sources & verification

ReferenceVerified
Ford U0121: built on or bef | EV Diagnostics