Petarded is episode 06 of family guy season 04. It was originally aired on June 19, 2005.
At a game night among friends, Lois lets Peter win at Trivial Pursuit, which leads Peter to gloat and think he’s a genius. Brian knows the truth and challenges Peter to prove his intelligence by getting an IQ test. But when the test results determine Peter is legally mentally challenged, everyone ends up looking stupid.