Fernando Alonso was back on Honda's Spec 2 engine on Saturday in Austria after an issue was detected with his MGU-H.
As the drivers took to the track for Saturday's third and final practice session at the Red Bull Ring, news broke that Alonso was running the older spec Honda engine.
The Spaniard had been using Honda's updated Spec 3 unit on Friday but was forced to revert overnight when an issue was detected with the MGU-H.
According to reports, Honda's Spec 2 is between 10 and 26bhp down on the updated version.
Stoffel Vandoorne remains on the Spec 3 unit.
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