Lyth was the leading English run-scorer in the LV= Insurance County Championship heading into the fourth round, with his 488 haul sparking some discussion over a Test recall following six years away
But the left-hander was bowled off his pad as he shouldered arms at seamer Sanderson, whose two wickets backed up superb South African Parnell in Yorkshire’s 206 all out.
Left-arm pacer Parnell finished with five for 64 in his second game for Northants and Dom Bess top-scored with 56 as 19 overs were lost to bad light and rain. Northants closed on a confident 36 without loss from 11 overs.
Sheffield-born Sanderson started his career at Headingley before being released in 2011.
Yorkshire slipped from 76 for two to 80 for five in the early stages of the afternoon
Ricardo Vasconcelos, deputising as captain and wicketkeeper for injured Adam Rossington, had won the toss under a bright sky before conditions became unsettled.
Only three balls were bowled between 12.30pm and 2.50pm
Tom Kohler-Cadmore’s 42 was his highest score of the season, with his form in stark contrast to that of opening partner Lyth’s.
Will Fraine - a replacement in Yorkshire’s side for the rested Joe Root - was lbw for 11 to Gareth Berg as the hosts reached 67 for two when lunch was taken early.
Yorkshire were plunged into trouble quickly after the 2.50pm resumption, losing Gary Ballance for four, Harry Brook for a duck and Kohler-Cadmore inside four overs.
Parnell had Ballance caught at first slip by Ben Curran via a rebound from Rob Keogh at second before Brook was trapped lbw driving at the same bowler.
Berg then bowled Kohler-Cadmore with a beauty of a delivery which nipped away slightly - evidence of the help on offer all day for bowlers - as Yorkshire fell to 80 for five after 27 overs.
Pugnacious Bess shared 57 for the sixth wicket with wicketkeeper Jonny Tattersall either side of tea.
Their partnership was broken by Sanderson when he had Tattersall caught at third slip for a season-high 26 in the 47th over.
Parnell then bowled Jordan Thompson and had David Willey and Duanne Olivier caught behind. Sandwiched in between, Berg had Bess caught at midwicket to finish with three for 32.
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