This article will help you to diagnose the Pulse Width Modulated (PWM) Fan Relay on the 1999-2004 Jeep 4.0L Grand Cherokee SUVs and/or the Codes P1491 (Radiator Fan Control Relay Circuit). The dealer, at the time of this writing, will charge you $95 (US) to diagnose it, but if you have more time than money, you can easily diagnose it yourself, and I’ll show you how.