My dads truck has a P0720 code. It's '99 CTD.
It says "low output speed sensor RPM, above 15mph".

Would this be the sensor on the rear diff going south?

 

yes I believe so..................cheap enough to replace to find out

 

yeah i did mine in the middle of winter axle all frozen up it sucked
but yeah rear diff passenger side top

 

Great guys, thanks for the confirmation I'll toss one in there for him and see how she runs.

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I found some more info on this code. it refers to the output speed sensor which is located on the tranny overdrive gear case and it monitors the trans output shaft rotating speed.
$30 at napa for the sensor.
hopedully that does it.