I own a 2005 Mazda 3 (SP 2.3 Manual) and I took the car to get the annual emissions test. During the OBD part of the test, my CEL came on, and the test listed a code of U0121.

As far as I can tell from my searching the internet, all the code means is that the test itself can't communicate with the ABS system.

A couple days after the test, the CEL dismissed itself, with no action on my part.

2 Questions:

  1. Am I correct in thinking that replacing a fuse would fix the U0121 code? If not, what is the right solution for that code?
  2. Given that the CEL has dismissed itself, if I were to go back to get another emissions test, would it pass without any further action on my part?

That code indicates a failure in communication between the ABS module and the PCM. It can be caused by failure of either one the computers, the wiring between the two or it can be caused by a scan tool used to check the system. The latter sounds most likely in this case since it came on during the test and cleared later on its own. A fuse is an unlikely cause of this failure.

In most testing jurisdictions if the CEL is off the system will pass the test unless codes have be cleared with a scanner recently.

If the testing process turns the light on during the test bring that to the attention of the test authority and have them check their equipment for defects.

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