On the 2007 GMC Yukon Denali, the P1825 points to a problem with the Transmission Mode Switch. The IMS is part of a wire harness, which connects electrically to the control solenoid valve assembly. It is serviced separately from the control solenoid valve assembly. Inspect the IMS, harness, connector, and control solenoid valve assembly pins for metallic debris. Check for an incorrectly adjusted range selector lever cable. Visually inspect the manual shaft detent assembly for proper alignment with the manual shaft detent lever. The manual shaft detent roller must be centered over the detent lever and not contact the IMS actuator arm. If there are no problems found, replace the Mode Switch. The Mode Switch is located on the valve body. To gain access, the transmission fluid pan will need to be removed.
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