The Toronto Transit Commission is not promising an end to all hot subway cars in Toronto but its boss says passengers can expect much a cooler ride when they take Line 2 this summer.

"We are pretty confident that this year there may be the odd hot car, but it certainly won't be the problem we had last year," TTC CEO Andy Byford told CBC's Metro Morning on Tuesday.

Byford said the TTC has repaired the air conditioning units on the majority of its hot subway cars. He said the "comfort of customers" on Line 2, the Bloor-Danforth line, will be greatly improved this summer now that the number of hot cars has been reduced.

"We have put a huge amount of effort since we had the problems last year. To be fair, we were already addressing the problems," Byford said.

TTC Chief Operating Officer Mike Palmer, TTC CEO Andy Byford and TTC chair Josh Colle, left to right, talk to reporters at Greenwood Yard. (CBC) "The problem was, we had so many hot days in a row, very hot days in a row, that we were overwhelmed. We could not fix the cars quickly enough and more cars failed than we could fix overnight."

At a news conference on Tuesday morning, the TTC conducted a stress test on a subway car to show reporters how one of its "newly overhauled" heating, ventilation and air conditioning systems withstood heat in a climate-controlled room built at Greenwood Yard.

Hot air was pumped into the room, which Byford called a "hot house." The subway car stayed cool inside despite it being more than 30 C outside.

Byford told reporters there were 35 days of high heat, many of which were consecutive, last summer and the TTC knew it was "failing " its customers.

"Our guarantee to customers is we will not have the widespread problems that we faced last year. We are absolutely determined to give our customers on Line 2 a fabulous, cool, trouble free journey," he said at the news conference.

Mike Palmer, TTC chief operating officer, looks at a gauge while the TTC conducts a stress test of a subway car to show how its air conditioning unit can withstand high heat. (CBC)

In the Metro Morning interview, Byford said the refurbishment of air conditioning units took place over the winter and spring and it involved hiring more engineers.

"We have recruited extra people and we have put a lot of resource onto completely renovating the air conditioning units on what are called T1 trains."

Byford said T1 trains, unlike the Toronto Rocket trains on Line 1, are older and each subway car has one air conditioning unit. The TTC has 370 T1 cars.

"You have to make sure those things are working. We have progressively worked through the fleet," he said.

Byford said the theme of his tenure has been "to get the basics right." Providing customers with a comfortable ride on hot days is part of that mandate, he said.

"We let our customers down. There was no question about it," he said.

Mayor John Tory and Bianca Spence emerge from Kipling subway station after riding in a car without air conditioning on a sweltering day last summer. (Linda Ward/CBC) TTC customer satisfaction scores dropped 10 points last summer, dropped from 80 to 70, but it's now back up to 79, he said. He attributed the drop in part to hot subway cars on Line 2.

Byford said the TTC hopes to replace the older cars on Line 2 eventually.

Mike Palmer, TTC chief operating officer, told reporters at the news conference that the T1 cars are 15 years old and the TTC is spending $13.5 million over two years to bring their air conditioning units up to date.

On a September day last year, Toronto Mayor John Tory rode Line 2 from Kennedy to Kipling stations with TTC rider Bianca Spence, who issued a Twitter challenge to the mayor in July after enduring sweltering subway rides during her commute.

Tory said at the time that his ride on a subway car without air conditioning was "a perfect storm" of challenges that gave him a sense of what a commute can be like for a TTC rider.

Last summer, the air conditioning on nearly 25 per cent of subway cars was either broken outright or not functioning at full capacity. The TTC had said the cars could not be taken out of service all at once because all of the cars were needed during peak service periods.