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Mazda U0420

MAZDA RADAR CRUISE CONTROL (MRCC)/SMART BRAKE SUPPORT (SBS)○ Multiple DTC's (U0XXX:00 COMMUNICATION ERROR)○ • If an aftermarket device is found connected to DLC-2, take a photo and

DriveNot yet established by verified evidence.
ChargeNot yet established by verified evidence.
ServiceContact a Mazda dealer or qualified Mazda service facility for normal MGSS diagnosis, particularly if no aftermarket DLC-2 device is installed or the code persists after its removal.
Confidence95%

Quick answer

U0420:00 is identified in Mazda service communications as a Mazda Radar Cruise Control (MRCC)/Smart Brake Support (SBS) communication-related DTC. Mazda bulletins specifically instruct dealers to check whether an aftermarket device is connected to DLC-2; if present, document it and advise its removal. If none is installed, Mazda directs normal MGSS diagnosis.

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 U0420 mean?

U0420:00 is identified in Mazda service communications as a Mazda Radar Cruise Control (MRCC)/Smart Brake Support (SBS) communication-related DTC. Mazda bulletins specifically instruct dealers to check whether an aftermarket device is connected to DLC-2; if present, document it and advise its removal. If none is installed, Mazda directs normal MGSS diagnosis.

2. Technical explanation

Verified Mazda/NHTSA bulletin evidence lists U0420:00 under MRCC/SBS and in the context of multiple U0XXX:00 communication-error DTCs. The supplied evidence does not establish a more specific component failure or root cause for U0420 by itself.

3. What system is affected?

Mazda Radar Cruise Control (MRCC) / Smart Brake Support (SBS), with communication-error DTC context.

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

  • An aftermarket device connected to DLC-2 is specifically addressed in Mazda service bulletins.
  • If no aftermarket device has been installed, the underlying cause is not yet established by the supplied verified evidence; Mazda directs normal MGSS diagnosis.

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

  • Check whether an aftermarket device is connected to DLC-2.
  • If an aftermarket device is present, Mazda bulletin guidance is to have it removed.
  • Arrange Mazda service if U0420:00 remains present or if no aftermarket DLC-2 device is installed.

10. Step-by-step troubleshooting

  1. A Mazda technician should determine whether an aftermarket device is connected to DLC-2.
  2. When such a device is found, Mazda bulletin guidance says to photograph it, attach the photo to the repair order, and advise the customer to remove it.
  3. When no aftermarket device has been installed, Mazda directs technicians to continue normal MGSS diagnosis.

11. When to contact service

Contact a Mazda dealer or qualified Mazda service facility for normal MGSS diagnosis, particularly if no aftermarket DLC-2 device is installed or the code persists after its removal.

12. What the service center may check

  • Inspect DLC-2 for an aftermarket device.
  • Document any aftermarket DLC-2 device found, including a photo attached to the repair order.
  • Continue normal MGSS diagnosis when no aftermarket device is installed.

13. Possible repair or resolution

  • Removal of an aftermarket DLC-2 device is specifically advised by Mazda when one is found.
  • Any further repair resolution is not yet established by verified evidence and depends on normal MGSS diagnosis.

14. Affected vehicles

15. Related TSBs / service bulletins

  • Mazda service communication SA-041-26, published June 19, 2026 (NHTSA MC-11034447), lists U0420:00 in the MRCC/SBS multiple-U0XXX communication-error context and addresses aftermarket devices connected to DLC-2.
  • Mazda service communication SA-061-25, published December 17, 2025 (NHTSA MC-11026917), lists U0420:00 in the MRCC/SBS multiple-U0XXX communication-error context and addresses aftermarket devices connected to DLC-2.
  • Mazda service communication SA-057-24, published August 15, 2024 (NHTSA MC-11007019), lists U0420:00 in the MRCC/SBS multiple-U0XXX communication-error context and addresses aftermarket devices connected to DLC-2.
  • Mazda service communications SA-032-23 and SA-032-23a (NHTSA MC-10237216 and MC-10244679) state that if an aftermarket device is found connected to DLC-2, the technician should photograph it, attach the photo to the repair order, and advise removal; if none is installed, continue normal MGSS diagnosis.

16. Recall information

  • No recall information is established by the supplied verified evidence.

17. Official manufacturer information

Applicable verified manufacturer communications are published through NHTSA: https://static.nhtsa.gov/odi/tsbs/2026/MC-11034447-0001.pdf ; https://static.nhtsa.gov/odi/tsbs/2026/MC-11026917-0001.pdf ; https://static.nhtsa.gov/odi/tsbs/2024/MC-11007019-0001.pdf ; https://static.nhtsa.gov/odi/tsbs/2023/MC-10244679-0001.pdf ; https://static.nhtsa.gov/odi/tsbs/2023/MC-10237216-0001.pdf

19. Frequently asked questions

What does Mazda DTC U0420:00 refer to?

The verified Mazda bulletin evidence identifies U0420:00 with Mazda Radar Cruise Control (MRCC)/Smart Brake Support (SBS) and multiple U0XXX:00 communication-error DTCs.

Can an aftermarket OBD/DLC device matter?

Yes. Mazda service bulletins specifically instruct technicians to check for an aftermarket device connected to DLC-2 and advise removal if one is found.

What if no aftermarket device is connected to DLC-2?

Mazda directs technicians to continue normal MGSS diagnosis. The supplied evidence does not establish a specific further cause or repair.

Which vehicles are covered by the supplied applicability evidence?

US-market Mazda CX-70 model years 2022–2026, CX-90 model years 2022–2026, and MX-30 model years 2022–2026 are listed in the supplied applicability records. Exact bulletin applicability should be confirmed by VIN and the applicable Mazda service information.

20. Sources & verification

ReferenceVerified
Mazda U0420: MAZDA RADAR CRUISE CONTROL (MRCC)/SMART BRAKE SUPPORT… | EV Diagnostics