Chevrolet U18B4
• Does the customer have an active OnStar account (With data available as applicable) ? • What happens when an OnStar® key press is made? • Does the system connect on a key press?
Quick answer
U18B4 is documented by Chevrolet/GM service information as a diagnostic code that may indicate an Ethernet issue and is associated with OnStar connection checks. The verified evidence applies to US-market 2020 and 2022 Chevrolet Bolt EV vehicles.
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 U18B4 mean?
U18B4 is documented by Chevrolet/GM service information as a diagnostic code that may indicate an Ethernet issue and is associated with OnStar connection checks. The verified evidence applies to US-market 2020 and 2022 Chevrolet Bolt EV vehicles.
2. Technical explanation
Official GM communication instructs technicians that, when diagnosis leads to U18B4 (or B101D), the condition may indicate an Ethernet issue. GM Technical Assistance Center (TAC) may verify the original customer concern and ask whether the customer has an active OnStar account, what occurs when an OnStar button is pressed, whether the system connects, and whether OnStar has a current GPS location after a button press. A separate official communication states that software updates improved Ethernet DTC diagnostic algorithms for U18B4, U1008, and U0078.
3. What system is affected?
OnStar-related connectivity and Ethernet diagnostic functions are supported by the verified evidence. A more specific component-level system identification is not yet established by verified evidence.
4. How serious is this code?
Unknown. The supplied verified evidence does not assign a severity level to U18B4.
5. Symptoms you may notice
- An OnStar concern may be relevant, including whether an OnStar key press connects and whether a current GPS location is available after a key press.
- Specific customer-visible symptoms are not otherwise established by verified evidence.
6. Possible causes
- An Ethernet issue may be indicated when diagnosis leads to U18B4, according to the official GM communication.
- Other causes are not yet established by verified evidence.
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
- Record what happens when an OnStar button is pressed.
- Check whether the vehicle has an active OnStar account, where applicable.
- Note whether the system connects after an OnStar button press.
- Note whether OnStar has a current GPS location after an OnStar button press.
- Have a Chevrolet/GM service provider diagnose the DTC and retain the DTC sub-code if present.
10. Step-by-step troubleshooting
- GM service information says TAC may need to verify the original customer concern that led to discovery of U18B4.
- For diagnosis involving U18B4, document the OnStar account status where applicable, the result of an OnStar key press, whether the system connects, and whether a current GPS location is available.
- The official communication states that the DTC sub-code must be recorded; the supplied evidence gives B101D as an example format but does not provide a U18B4 sub-code example.
- Do not attempt high-voltage or network-module repair procedures without qualified service information and training.
11. When to contact service
Contact a Chevrolet/GM dealer or qualified GM service provider if U18B4 is present or if an OnStar connection concern is occurring. GM TAC involvement may be part of dealer diagnosis.
12. What the service center may check
- Confirm and document the original customer concern.
- Retrieve U18B4 and record its sub-code, if displayed by the diagnostic equipment.
- Verify active OnStar account status where applicable.
- Perform and document an OnStar key-press connection check.
- Determine whether OnStar reports a current GPS location after a key press.
- Review applicable GM software updates, since official GM communication reports improved Ethernet DTC diagnostic algorithms for U18B4.
13. Possible repair or resolution
- A specific repair is not yet established by verified evidence.
- Official GM communication documents software changes that improve Ethernet DTC diagnostic algorithms for U18B4, but the supplied evidence does not establish that a software update resolves every U18B4 occurrence.
14. Affected vehicles
- Chevrolet Bolt EV · 2020 · US
- Chevrolet Bolt EV · 2022 · US
15. Related TSBs / service bulletins
- NHTSA Manufacturer Communication 10172292, document 030846004W, published January 13, 2020: States that U18B4 may indicate an Ethernet issue and provides OnStar-related information requested by GM TAC during diagnosis.
- NHTSA Manufacturer Communication 10215627, bulletin 21-NA-188, published June 24, 2022: States that software updates improved Ethernet DTC diagnostic algorithms for U1008, U18B4, and U0078.
16. Recall information
- No recall information was supplied in the verified evidence.
17. Official manufacturer information
Verified official sources: NHTSA-hosted GM manufacturer communications 10172292 (030846004W) and 10215627 (21-NA-188).
19. Frequently asked questions
What does U18B4 mean on a Chevrolet Bolt EV?
The supplied GM service evidence says U18B4 may indicate an Ethernet issue and directs diagnosis toward documenting OnStar account, button-press connection, and GPS-location results.
Which Bolt EV model years are supported by the supplied evidence?
US-market 2020 and 2022 Chevrolet Bolt EV vehicles.
Does U18B4 mean the vehicle is unsafe to drive or charge?
Not yet established by verified evidence.
Is there an official update related to U18B4?
Yes. GM bulletin 21-NA-188 states that software updates improved Ethernet DTC diagnostic algorithms for U18B4, U1008, and U0078. Whether an update applies to a particular vehicle must be confirmed by GM service.