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Cadillac U1666

voltage battery surge tank and document via photo. 3. Observe and record the Battery Energy Control Module parameter(s); K16– Battery Energy Control Module -> Data Display -> High

DriveNot yet established by verified evidence.
ChargeNot yet established by verified evidence.
ServiceService by a technician who has successfully completed the required training is indicated in the supplied Cadillac bulletin. The bulletin states that repairs must be performed by such a technician.
Confidence95%

Quick answer

U1666 is included in Cadillac bulletin 23-NA-151 diagnostic criteria for certain U.S. Cadillac LYRIQ and VISTIQ vehicles. The supplied evidence does not establish a standalone plain-language definition for U1666.

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 U1666 mean?

U1666 is included in Cadillac bulletin 23-NA-151 diagnostic criteria for certain U.S. Cadillac LYRIQ and VISTIQ vehicles. The supplied evidence does not establish a standalone plain-language definition for U1666.

2. Technical explanation

Cadillac bulletin 23-NA-151 instructs trained technicians to verify whether U1666 is current as part of a high-voltage battery diagnostic process. The bulletin also calls for documentation of high-voltage battery surge-tank coolant level and review of Battery Energy Control Module high-voltage isolation data. This does not establish that U1666 alone identifies a specific failed part or condition.

3. What system is affected?

High-voltage battery diagnostic process; Battery Energy Control Module high-voltage isolation data; high-voltage battery surge tank coolant-level documentation.

4. How serious is this code?

Not yet established by verified evidence.

5. Symptoms you may notice

Not yet established by verified evidence.

6. Possible causes

  • Not yet established by verified evidence. The supplied bulletin identifies U1666 as a diagnostic criterion but does not establish a specific cause for U1666 by itself.

7. Can I still drive the vehicle?

Not yet established by verified evidence.

8. Can I charge the vehicle?

Not yet established by verified evidence.

9. What you can safely do

  • Arrange diagnosis by a Cadillac/GM service facility or technician trained for the required high-voltage battery work.
  • Do not attempt high-voltage battery diagnosis or repair yourself.

10. Step-by-step troubleshooting

  1. A trained technician may verify whether U1666 is current, as specified in Cadillac bulletin 23-NA-151.
  2. The bulletin directs the technician to visually inspect and document coolant level in the high-voltage battery surge tank with a photo.
  3. The bulletin directs the technician to observe and record Battery Energy Control Module high-voltage isolation-data parameters.

11. When to contact service

Service by a technician who has successfully completed the required training is indicated in the supplied Cadillac bulletin. The bulletin states that repairs must be performed by such a technician.

12. What the service center may check

  • Verify whether U1666 is set as current, following the applicable version of Cadillac bulletin 23-NA-151.
  • Document coolant level in the high-voltage battery surge tank by photo.
  • Review and record Battery Energy Control Module high-voltage isolation data, including the most recent isolation-resistance information where applicable.
  • Follow the complete applicable bulletin procedure and any required data-capture process.

13. Possible repair or resolution

  • Not yet established by verified evidence. The supplied evidence provides diagnostic and documentation steps, not a universal repair for U1666.

14. Affected vehicles

15. Related TSBs / service bulletins

  • Cadillac/GM bulletin 23-NA-151 is documented through NHTSA manufacturer communications, including MC-10243714 (September 5, 2023), MC-10245234 (October 19, 2023), MC-11002022 (June 5, 2024), MC-11011944 (November 20, 2024), MC-11012693 (December 11, 2024), and MC-11017886 (April 9, 2025).
  • The versions supplied list U1666 among diagnostic trouble codes considered during the bulletin procedure. Bulletin revisions vary in their listed DTCs and details.

16. Recall information

  • Not yet established by verified evidence. No recall information was supplied.

17. Official manufacturer information

Official source: NHTSA-hosted Cadillac/GM manufacturer communications for bulletin 23-NA-151, including https://static.nhtsa.gov/odi/tsbs/2025/MC-11017886-0001.pdf

19. Frequently asked questions

What does U1666 mean on a Cadillac?

The supplied verified evidence does not establish a standalone definition. It establishes that U1666 is included in Cadillac bulletin 23-NA-151 diagnostic criteria.

Which vehicles are covered by the supplied applicability data?

U.S. Cadillac LYRIQ model years 2023–2026 and U.S. Cadillac VISTIQ model years 2023–2026 are listed in the supplied applicability data. Some entries list shorter LYRIQ year ranges; the supplied data collectively extends LYRIQ through 2026.

Is U1666 proof that the high-voltage battery must be replaced?

No such conclusion is established by the supplied evidence. The bulletin provides inspection, documentation, and data-review steps rather than a universal repair instruction.

20. Sources & verification

Verified
Cadillac U1666: voltage battery surge tank and document via photo. 3.… | EV Diagnostics