Toro Rosso have fitted older-spec Honda engines for the Mexican GP due to fears over the upgraded version, but Pierre Gasly is still set to start at the back of the grid again.
Both Gasly and Brendon Hartley ran Honda's 'Spec Three' power unit at the two previous races and appeared to receive a boost in performance, but the Japanese supplier say they detected a fault in Gasly's engine and don't want to risk their latest versions in Mexico's high altitude.
Honda did, however, introduce a new PU for Gasly in Practice One only, to add "to the pool of available units for the rest of this season" in case his Austin engine can't be salvaged.
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Gasly didn't post a lap time in the session, but as he is now up to eight components for the year he will take a 15-place penalty and looks certain to start towards the back of the grid for the third time in four races - even though he will revert back to an older engine from Practice Two onwards. Hartley is running the older spec all weekend.
"The reason for reverting to the earlier pre-Russia spec is that, we have a better understanding of this version of the PU and are more confident in our ability to set it up correctly for the specific conditions here in Mexico City," said Honda technical chief Toyoharu Tanabe.
Toro Rosso chief Franz Tost continued: "We did two laps, decided to change the power unit to the old specification, only because of this very special altitude here. Honda thinks they have better knowledge and experience with their older specification to set all the parameters correctly and therefore Pierre will do the race weekend with Spec Two."
Honda start their partnership with Red Bull next season and there was a lot of anticipation for their Spec Three engine, despite Toro Rosso opting against using the unit in Russia after an unsuccessful practice debut.
The refined engine was thought to be worth half a second when it was brought to Japan and both Gasly and Hartley reached Q3, before starting at the back last weekend in Texas after Honda's "minor modifications".
But the latest Mexico setback was unexpected, with the circuit's high altitude conditions complicating matters.
Tost added: "Regarding the work of Honda during the year, they have so far done a fantastic job because they have improved the performance a lot, and also the reliability, and I am really looking forward to the last two races, in Sao Paulo and Abu Dhabi, because there hopefully we won't have any penalties.
"And for next year, because they are in the right way and they will improve during the winter months and I'm looking forward to seeing Red Bull Racing and Toro Rosso next year with Honda."
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