Nichols, who once scored for Posh against Cobblers at London Road, struck four minutes before the half-time interval.
On-loan Posh goalkeeper Christy Pym made his debut in a 3-1 win for Stevenage against Walsall. He should have done better with the Walsall goal.
In the third round of the FA Cup former Posh full-back Alex Penny was part of non-league Kidderminster’s team in a 2-1 win over Championship side Reading, while Alex Woodyard was in the Wimbledon side beaten 2-0 at National League Boreham Wood.
Ex-Posh boss Mark Cooper’s Barrow side lost a remarkable game 5-4 at Championship strugglers Barnsley. Barrow had a man sent off in the first-half at 1-0 down, but fought back from 2-0, 3-2 and 4-3 down to force extra time.
Grant McCann’s Hull City lost a cracker 3-2 at home to Everton.