The visitors made a bright start and created several good openings. Etienne Esajas fired over from long range, Sam Winnall was unable to capitalise on a defensive slip and Eddie Nolan flashed a shot wide from the edge of the box.
But Niall Canavan missed Scunthorpe's best chance when he failed to beat Matt Duke from six yards after being picked out by Matt Sparrow's pin-point free-kick.
Northampton failed to create any chances against a well-drilled Scunthorpe defence and Chris Iwelumo headed wide from Esajas' corner after the restart as the Iron picked up where they left off.
Canavan then had a header saved by Duke from Terry Hawkridge's corner and Winnall was also thwarted by the Cobblers goalkeeper.
Platt tested Sam Slocombe with a header as Cobblers finally got going and substitute Jacob Blyth was just off target with a far post header from Chris Hackett's deep cross.
But it was Scunthorpe who broke the deadlock in the 67th minute when Canavan rose well to head home Hawkridge's deep corner.
Darren Carter went close to an equaliser when he broke away but Slocombe tipped aside his effort and saved from Hackett's follow-up.
But Cobblers equalised in the 78th minute when Scunthorpe failed to deal with Lee Collins' searching pass and Platt nipped in to slip the ball under Slocombe.
Winnall went close to a winner late on while, at the other end, Hackett's inswinging corner almost caught out Slocombe as the ball came back off the upright.