Both of Pachall's scoring throws came in the first half and the Horned Frogs (4-2, 2-0 Mountain West) took a 20-0 lead in the third quarter.
The Aztecs (3-2, 0-1) came within 20-14 in the second half on a pair of touchdown passes by Ryan Lindley, but they got no closer.
Pachall completed 14 of 22 passes for 212 yards with two interceptions as TCU outgained San Diego State, 446-296, including 234 yards on the ground.
TCU set a Mountain West record with its 19th consecutive conference win, breaking a tie with BYU. It was also the Horned Frogs' 15th win in the last 17 road games.
Antoine Hicks caught a 39-yard TD pass with 23 seconds left in the first quarter to give the Horned Frogs a 10-0 lead. Josh Boyce then made a 31-yard catch for a touchdown at the 6:14 mark of the second quarter to help TCU to a 17-0 halftime lead.
San Diego State lost a pair of scoring opportunities in the first half. The first came when holder Brian Stahovich muffed the snap on Abel Perez's 38-yard field goal attempt. The next was on Ronnie Hillman's fumble at the 1-yard line that was recovered by the Horned Frogs in the end zone.
Ross Evans added field goals of 47 and 21 yards for TCU, which bounced back from last week's 40-33 overtime loss to SMU that knocked the Horned Frogs out of the Top 25.
Ed Wesley rushed for 146 yards on 24 carries for TCU and Matthew Tucker had 69 yards, including a 4-yard TD run with 4:45 left to seal the win.
San Diego State got on the board in the third quarter following a muffed punt by Skye Dawson that was recovered by Vaness Harris at the TCU 15. Three plays later, Lindley hit Gavin Escobar with a 16-yard touchdown pass at the 5:29 mark.
Lindley also threw a 4-yard scoring pass to Chad Young early in the fourth quarter to cut the TCU lead. But Lindley threw his third interception on the Aztecs' next possession that led to TCU's final score.
Lindley completed just 15 of 41 passes for 201 yards and two interceptions. Hillman had his six-game streak of 100-yard games snapped as he rushed for 55 yards on 20 carries.