Coach Gary Patterson announced the suspension hours after police say Pachall was pulled over for running a stop sign near TCU's Fort Worth campus.
"My job as a head coach is to win games, educate our kids and help them with their lives," Patterson said in a statement.
Pachall's "breath specimen" was over the legal limit (0.08), said Pedro Criado, Fort Worth police spokesman. Pachall was transferred to the Mansfield (Texas) jail, where he was later released on $1,000 bond.
Pachall, 21, lived in the same house as former TCU linebacker Tanner Brock, who was among four Horned Frogs football players arrested in February during a campus-wide drug bust. Pachall admitted to drug use in police interviews, but was not arrested.
TCU confirmed that Pachall had failed a Feb. 1 drug test, just two weeks before the arrests. Pachall attended a university drug and alcohol awareness program and was not suspended for any game time by Patterson.
"I've got a young football team and a person who obviously needs help," Patterson told The Associated Press. "Until I can come up with some answers, it's not about one season, it's about a lot of seasons."
Pachall, a junior, started all 13 games for TCU a year ago. He has 948 passing yards and 10 touchdowns this season for the 4-0 Horned Frogs, who are ranked No. 15. TCU is set to host Iowa State on Saturday.
TCU enters this week having won 12 straight games, the longest active streak in the FBS. According to ESPN Stats & Information, TCU's eight remaining opponents have a .792 win percentage, tied for the best in the FBS. The Horned Frogs' first four opponents had a combined .143 win percentage, one of the lowest in the FBS.
Redshirt freshman Trevone Boykin is TCU's likely starter at quarterback Saturday. Boykin has played in three games this season, completing 9-of-10 passes for 74 yards and a touchdown. He has 12 rushes for 122 yards.
Before Pachall's arrest, there was anticipation that Boykin could see action at tailback against Iowa State since Matthew Tucker is dealing with an ankle injury. The Frogs already had lost leading rusher Waymon James to a season-ending left knee injury suffered at Kansas three weeks ago.
"This is not about this season," Patterson told the AP. "This about a young man's life, about a whole bunch of kids."
Also Thursday, the last of four former TCU football players arrested in a drug sting earlier this year pleaded guilty to marijuana delivery charges and received probation.
The four were among more than a dozen students netted by the sting.
Tarrant County district attorney's spokeswoman Melody McDonald said Thursday that Devin Johnson entered his plea in August and also was ordered to pay a $300 fine and court costs.
Former players Brock, D.J. Yendrey, and Tyler Horn pleaded guilty a few weeks before Johnson and received similar sentences.
The arrests came in February after a six-month drug investigation by Fort Worth police acting on a tip that a player was selling marijuana to other players and students.
Information from The Associated Press was used in this report.