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I found different versions in the English and German Wikipedia articles about the Apollo 6 flight.

"Das zweite Triebwerk wurde vom Steuerungssystem fälschlicherweise deaktiviert, weil die Steuerungsleitungen zwischen den beiden Triebwerken vertauscht waren."

Translation: The second engine was shut down erroneously due to swapped control cables between the two engines.

The English version is just about two independent engine shut downs.

I found no other sources confirming the German version. Such a cabling error would be difficult to prove using telemetry data only. But what else could prove the error after the flight? Is there any other information about the Apollo 6 engine shut down?

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The official NASA history of the Saturn vehicles "Stages to Saturn", pp. 361-362 agrees that there was a miswired control cable.

..the low-thrust sensing equipment triggered a sequence to shut down the engine by closing the fuel and oxidizer valves. The electrical sequence to close number two LOX valve went erroneously to number three. Closing the fuel valve for engine number two and the oxygen valve for engine number three shut down both engines.

(The second occurrence of "sequence" appears to be an error; it should probably read "signal".)

I have the print version of the book, but in the online one it's on this page (look for the [362]).

This chapter also discusses the ground investigation via the telemetry data in some detail. The root cause was a rupture of the igniter propellant lines due to resonant vibration. Ground testing had not identified the failure mode because when the engines were fired in an atmosphere, air that liquefied on the lines dampened the oscillation enough to prevent failure.

There is an extremely detailed analysis of the failure in this document, which I just noticed in the excellent answer to this question: How common is the ability to compensate for a lost engine through gimbaling?

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    $\begingroup$ "Finally, Rocketdyne technicians decided to test the lines in a vacuum chamber, in close simulation of the environment where failure occurred. Eight lines were set up for test in a vacuum chamber, and engineers began to pump liquid hydrogen through them at operational rates and pressures. Before 100 seconds elapsed, each of the eight lines broke; each time, the failure occurred in one of the bellows sections. By using motion picture coverage acquired during repeated vacuum chamber tests, Rocketdyne finally could explain the failures." Only tests in a vacuum could reproduce the rupture. $\endgroup$ – Uwe Dec 7 '17 at 22:04
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    $\begingroup$ The old and new propellant lines are shown in this picture on the right. The old lines with bellows sections, the new without. The liquid air damping of bellows vibrations is shown here. Images are part of this page. $\endgroup$ – Uwe Dec 8 '17 at 21:28
  • $\begingroup$ The resonant vibration rupture problem did not show during the ground tests of the J-2 engine. The first flight of the Saturn V during the Apollo 4 mission was not influenced. The next Saturn V flight for Apollo 6 only one propellant line of five broke. The following first manned flight with a Saturn V for Apollo 8 (first flight to the Moon) used the modified lines without bellows sections. But during the ground test in a vacuum chamber all eight tested fuel lines did fail. There were three additional ground tests of the J-2 engine to verify the redesigned fuel lines. $\endgroup$ – Uwe Dec 8 '17 at 21:55
  • $\begingroup$ @Uwe ...and the previous tests had been done at sea level air pressure I guess? $\endgroup$ – Dave Gremlin May 10 at 19:29
  • $\begingroup$ @DaveGremlin yes, the previous engine tests had been done at sea level air pressure. The large J-2 engines used for the second and third stage could not be burn tested in a vacuum chamber. $\endgroup$ – Uwe Jun 20 at 8:57

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