Mercedes was denied a sixth consecutive Formula 1 one-two finish by the failure of the kinetic energy recovery system (ERS-K) on both of its cars in the Canadian Grand Prix.
Halfway through the race, both Lewis Hamilton and Nico Rosberg reported a loss of power. Their top speeds dropped and, as a result, so did their laptimes.
Hamilton soon retired with smoke pouring from the rear brakes, while Rosberg managed to hold on to the lead until he was passed by Daniel Ricciardo on the penultimate lap.
Mercedes has confirmed that a high-voltage control electronics failure led to the permanent loss of MGU-K driver on both cars.
Canadian Grand Prix race report
In simpler terms, the electronic units that switch the power from the battery to the kinetic motor generator unit (MGU-K) failed.
This would result in the drivers losing some 160bhp from the power unit, leaving only the conventional turbocharged internal combustion engine to propel the cars, with just over 600bhp.
Having to switch hundreds of volts between the MGU-K and the battery, the control electronics hardware has to run with high electrical loads and at high temperatures.
Control electronics have proven to be the most unreliable of the six modules of the new 2014 power units.
Mercedes has enjoyed better reliability with these components than its rivals, although Hamilton had replaced his control electronics after Saturday morning practice. That was the third unit he has used this year, compared to Rosberg's two.
With this failure, Hamilton will have to use his fourth set of control electronics, meaning he has just one more set to use for the balance of the season without incurring grid penalties.
As both units failed on the same lap and just after pitstops, it is likely that excessive heat may be a factor in the problems.
Mercedes blames overheating systems
With high ambient temperatures in Montreal and a spike in temperature, the car being stationary at a pitstop appears to have overcome the electronics.
Compounding the loss of the ERS-K, there is the impact this has on the brakes.
As the ERS-K harvests its energy, it also creates drag on the rear axle and acts as part of the car's braking system.
Without it, the car therefore loses a proportion of the rear braking effort.
To save weight, teams have downsized the rear brake disc and caliper for 2014. This in turn leaves the rear brakes unable to cope with delivering the entire braking effort alone.
With Montreal being one of the hardest tracks for braking, is not surprising that Hamilton's rear brakes failed soon after the MGU-K problem hit. That said, Paddy Lowe did suggest to Sky TV that the two were not related.
Rosberg adjusted his brake bias and, with a lack of braking effort and the absence of 160hp, still managed to finish second.
His early fuel consumption problems would have been exacerbated by having only the internal combustion engine to power the car for the balance of the race.