Stocks were in the red through to the end of the day on Tuesday after sales warnings from Caterpillar (CAT) - Get Report and Under Armour (UA) - Get Report spooked investors. Crude oil added to losses after closing down 1%, under $50 a barrel. Sprint (S) - Get Report slumped despite a better-than-expected quarter in which its postpaid phone churn reached a record low. The telecom narrowed quarterly losses and raised its full-year operating income target. DuPont (DD) - Get Report also enjoyed a positive quarter, posting adjusted earnings more than double the same period a year earlier. The chemicals company also boosted its full-year profit estimates. Apple (AAPL) - Get Report is reportedly testing out a car operating system which could operate self-driving vehicles. Apple reportedly has dozens of engineers working on the software in Canada.