Toyota has recalled millions of vehicles due to reports of sticking gas pedals and unintended acceleration. But finding out exactly what causes the problem has proven difficult. An explanation for why most of the accidents have occurred in the U.S. has likewise proven elusive.

It is an agonizing predicament that Toyota finds itself in - the most excruciating in the company's history. Vehicles accelerating on their own continue to cause problems, and the inability to bring the matter to a close could spell ruin for the company.

Worn down Toyota managers wanted to bring a little optimism to the Geneva Motor Show last week, but the latest bad news - that repairs failed to solve the car maker's gas pedal problem - ruined the mood.

Numerous Toyota drivers in the United States have alerted the National Highway Traffic Safety Administration (NHTSA), the country's highest transport authority, that their vehicles continue to accelerate by themselves even after repairs made following the recall.

The NHTSA isn't releasing details at the moment, but the news feeds a suspicion that has been circulating for some time - that the cause of the problem lies not with the pedal mechanism, nor is it due to floor mats sliding out of place. Rather, it is thought that the engine's electronic steering system is to blame. Were that to be the case, it would render pointless the repairs, which involve inserting metal shims under the gas pedals, currently being carried out in Toyota repair shops.