Recently (August 7), Booster 9 conducted a static fire test where it only ignited 29 of 33 engines. This is the big static fire, there not another one. I have found that they were shutdown prematurely a lot (example), but no where does it say why. So, what happened, and why were 4 engines shutdown prematurely?
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SpaceX has not released that information at this time, and may never do so. They have no strong reason to publicize it, really.
The engines will shut down automatically if they detect any of a wide variety of issues, so each engine shutdown could have been anything from combustion instability to a bad sensor that didn't report correctly under live conditions. A premature engine shutdown is a bit like a car's "Check Engine" light; it doesn't really tell us anything except "something went wrong".
I think it's more interesting that the test was meant to be 5 seconds but actually ended just short of 3 seconds. That suggests something happened halfway into the test that caused a full system halt.
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6$\begingroup$ " A premature engine shutdown is a bit like a car's "Check Engine" light; it doesn't really tell us anything except "something went wrong"." It may not tell us but I would bet that it tells SpaceX what went wrong. Even the 1970s vintage SSME controller sent failure identification codes to telemetry explaining why it shut the engine down. $\endgroup$ Commented Aug 8, 2023 at 22:09
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2$\begingroup$ The engine shutdown would tell SpaceX very little - they would need to consult additional data to make a conclusion. It's just that we don't have access to that. $\endgroup$– SteveCommented Aug 8, 2023 at 22:39
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1$\begingroup$ The launch commit criteria for the Starship system require 30 working engines. Assuming that SpaceX "tests as they fly", that would mean 4 engines out would be an automatic abort. $\endgroup$ Commented Aug 9, 2023 at 4:20
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$\begingroup$ @OrganicMarble Oh, sure, there's data SpaceX has access to that'll tell them exactly what happened; I assume they know precisely what was going on. For some failures we (outsiders) can speculate about possible causes even without official statements, because there's only a few things that could create the scenario we saw, but in this case it's too vague. $\endgroup$ Commented Aug 9, 2023 at 4:24