The defending world 500cc champion lost control of his Honda on a left- hand bend as he tried to pass a slower rider. He careered off the track, sped down a grass verge towards a perimeter wall before he hauled the bike back on to the tarmac.
Doohan thought the mistake happened as he pressed for a leading time. "Because I was getting a bit frustrated I turned on the gas rather aggressively and ran off the track," he said.
Doohan, aiming for his fourth consecutive world title and his seventh victory of the season, recorded 1min 49.755sec, 0.381 seconds slower than Checa, also on a Honda, who lapped Imola's recently remodified circuit in 1:49.374.
Anthony Gobert was third on a Suzuki in 1:50.096 with Nobuatsu Aoki, on a Honda, fourth.
Qualifying times, Digest, page 28