Mercedes has revealed the difficulties it endured with Lewis Hamilton on the second day of pre-season testing that left the seven-time F1 champion at the bottom of the timesheet.
Hamilton managed only 40 laps during the four-hour morning session at Barcelona's Circuit de Catalunya, with his time of one minute 22.562secs the slowest of the 16 drivers that took to the track across the day.
So far, Hamilton has only logged 90 laps in total across his two runs, considerably down on the likes of Red Bull's Max Verstappen, Ferrari pair Charles Leclerc and Carlos Sainz, as well as McLaren duo Lando Norris and Daniel Ricciardo.
Explaining the problems that blighted Hamilton on Thursday, trackside engineering director Andrew Shovlin said: "We had a difficult morning with Lewis, experiencing a data logging issue that delayed our start and in general, we were struggling a bit more with the balance.
"Some of that may have been set-up items that we were trying and some of it the harder tyres that we were running.
"We managed to get a few runs completed then, unfortunately, had a sensor issue that prevented us doing further performance work so we stopped early for lunch in order to fix it."
Mercedes at least had a more enjoyable afternoon with George Russell at the wheel as the Briton was fourth quickest with a time of 1:20.537s and clocking 66 laps.
Shovlin added: "George had a more productive session in the afternoon and we got through some useful set-up scans and completed some of our tyre programme.
"We've still got plenty of issues to work on from a performance point of view but we are continuing to learn with every run which in turn will make us faster.
"We've got a busy day planned for tomorrow, plenty more set-up work and a first look at some of the softer rubber compounds for this year."