Electrical gremlins held up Lewis Hamilton in Italian Grand Prix practice on Friday after the Mercedes driver had lapped more than half a second quicker than anyone else in the morning session.
The Briton, who retired from the race in Belgium 12 days ago after team mate and championship leader Nico Rosberg hit the back of his car, missed an hour of the afternoon running as mechanics replaced sensors on his car.
Hamilton was still fastest overall on the day, with a best time of one minute, 26.187 seconds set before lunch. Rosberg was top of the afternoon timesheets with a best of 1:26.225, just ahead of his team mate.
Hamilton is 29 points behind Rosberg
Jenson Button, Hamilton's former team mate at McLaren, was second fastest in the morning in 1:26.810 with Rosberg - who has never been on the Monza podium - pushed back to third in 1:26.995.
Hamilton is 29 points behind Rosberg in the championship with seven races remaining and is determined to claw his way back against his German rival, who was punished by the team for the Spa collision.
Ferrari's Fernando Alonso was fourth fastest in both sessions with Williams' Valtteri Bottas fifth in the second practice after a quiet morning. Button was sixth for McLaren, who last won a race in 2012.
Red Bull's quadruple world champion Sebastian Vettel, last year's winner at the 'cathedral' of Italian motorsport, was sixth and seventh while Australian team mate Daniel Ricciardo was 16th and 10th.
Merhi lapped faster than regular race
Ricciardo, as good as Italian to many of the home fans and chasing his third win in a row, was sidelined for some of the first session with a power unit problem.
At the slow end of the field, Spanish driver Roberto Merhi made his debut appearance at a Formula One weekend and lapped faster than regular race driver Marcus Ericsson in the other Caterham.
Merhi handed the car back to Japanese Kamui Kobayashi, returning after being dropped by the team for Spa, for the afternoon and rest of the weekend.
Spanish reserve driver Daniel Juncadella also had a stint with Force India while Frenchman Charles Pic was handed compatriot Romain Grosjean's Lotus for the session.