It appears that the shuttle fleet is grounded until they can resolve the problem with the Hydrogen engine cutoff sensor. (Paid subscription required)

Basically, if there is a problem and the engine runs out of hydrogen before getting shut down

A cutoff is essential before hydrogen depletion to avoid cavitation in the 39,000-rpm. main engine high-pressure fuel pumps that would result in an explosion.

So this is a MAJOR problem.

But wait, there’s more:

The space shuttle fleet is grounded until technicians find and fix a vexing problem with engine cutoff (ECO) sensors—a component that NASA now realizes has likely never worked throughout the 26-year history of the shuttle program.

…..So why do we need an LH2 cutoff system? Simply put, if you need it and the LH2 tank runs dry with the engines at full power with LO2 still coming in, inevitably a catastrophe will occur. LOX-rich shutoffs are ugly in the extreme. This is a crit 1 situation. And it would occur so rapidly that human intervention is not practical.”

What’s going on here is that the shuttle is in the end of life phase, so the pressure to cover-up problems is decreasing, and so we are seeing long term potentially catastrophic loss of vehicle, crew, etc.

The Shuttle has been a pig since day one, and NASA has seen their job as covering up problems so that they can continue to play Captain Kirk.