The reasons for this are as follows.
During Friday’s FP1, Buemi’s car had a fuel pressure problem, which then got worse at the start of FP2 which is why he did not do a timed lap in that session. After FP2 the entire fuel system on his car was changed. When the fuel sample was taken from his car after qualifying, it did not match the one provided to the FIA prior to the start of the season. The team believes that some part of the new fuel system contained a chemical that contaminated the fuel and caused the non-conformity.