The change-out of ET-119’s ECO (Engine Cut Off) sensors has been slightly delayed, due to issues relating to the replacement ECO sensors.
KSC (Kennedy Space Center) Project Engineering are understood to have called for the delay, in order to verify the new ECO’s won’t have the same “quality issue” as those being replaced – referring to an investigation into current stock of sensors.
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NASA has around six weeks of contingency time in the processing flow towards the launch of STS-121 in a window beginning July 1, with around 17-22 days of the contingency being used for the R&R of the ECO sensors – and TPS (Thermal Protection System) work on the aft dome of the tank.
The requirement to replace the ECO sensors came after a phase shift was noted on an LH2 sensor. NASA also had ECO sensor problems before the launch of Shuttle Discovery on STS-114 last year.
The last official date for the start of ECO change out was noted to be the 23rd of March, that is understood to have been moved to a NET (No Earlier Than) date of March 27. Should all go well with the ECO R&R, launch processing timelines project the mating of the ET and SRBs to commence on April 19. 
NASA has also decided not to give the go-ahead for the replacement of vent and relief valve on ET-119, at least for now, as more testing on the part is required.
ET-119’s V/R valve showed LOX tank pressure decreases, following shipping, raising some concerns that the part may be ‘a little leaky,’ according to sources.
A noon meeting at KSC today was set to make a decision to replace the V/R Valve.
However, following discussions, technicians will pressurise the valve and monitor the any off-limit changes in pressure levels, with the subsequent pressure data being analyzed for the decision on if to change-out and replace the valve.
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