Plans for Jenson Button to run the updated Honda engine at the Mexican Grand Prix have been put on hold until McLaren knows what went wrong with Fernando Alonso's power unit in Austin.
Alonso had been running strongly in the United States Grand Prix when his engine started losing power in the closing stages.
Although he managed to get to the finish, he had dropped out of the points by the time the chequered flag came out.
Button is scheduled to use in Mexico the upgraded engine that Alonso raced last weekend, but McLaren racing director Eric Boullier says he wants answers on its reliability before committing to that.
"Fernando lost power, but we don't know exactly why yet," said Boullier.
"We understand that there was an issue with some sensors maybe, so we had to tell him to reprogram differently the car, but the damage was done."
Boullier said it was too early to know if Alonso's issues in Austin meant he would need more components before the end of the campaign.
Button has had to wait to get hold of the engine, believed to deliver a 20bhp step over the old specification, while Honda manufactured enough new parts.
But Boullier suggests he will only go ahead with the switch if there are assurances that the power unit will last longer than the single race Alonso has used it for.
"We need the investigation on the engine before we decide," added Boullier. "Because we need the engine to last longer than this."