Toyota Gazoo Racing team principal Tommi Makinen says an ECU or fuel-related failure are the most likely reasons for Ott Tanak's retirement from day two of Rally Turkey on Saturday.
The Estonian failed to start the second stage of the day after his Yaris WRC refused to fire-up ahead of the Datca test.
As usual, Tanak stopped the car shortly before the arrival control for the stage and tried to re-start the Toyota as his due time into the control neared.
"We don't know yet, we wait for the car [to come back to service]," Makinen told Autosport.
"But I think ECU or something with the fuel, I don't think it can be anything else.
"We had this one time before when the car refused to start, but when the ECU cooled down it started - this was the worst thing to happen because then we couldn't find out what had happened at all. This time we want to find out."
Tanak confirmed Making's thoughts, saying: "I just came here and nothing happens. Something electrical. We stopped and nothing like this happened. There is no need to control everything now. It is back to the fight in Wales.
"It's difficult to fight for a championship if we have issues like that."
Tanak started Rally Turkey with a 33-point lead over Thierry Neuville and 40 points in hand over Sebastien Ogier.
Neuville slipped down the leaderboard when he went off the road this morning, while Ogier sits 2.2s behind Citroen team-mate Esapekka Lappi in second place.
Makinen added: "This keeps everything interesting for you, not so much for us though."
Asked about Neuville's demise, Hyundai Motorsport director Andrea Adamo told Autosport: "There's nothing to discuss. It's a matter of fact.
"He had a puncture in Germany and he crashed here. For him, the championship is not looking well now.
"Already before it was not very, now it's even worse. On the other hand, the rally's not over."