Owner's Problem Description
The owner reports that the 2004 Subaru Outback occasionally experiences issues with shifting gears. Specifically, when the Check Engine Light (CEL) and sport mode light illuminate simultaneously, the transmission fails to upshift. The only trouble code present is P1799, which is an OBD-I interlock code. The vehicle has not had significant repairs recently, and the owner is seeking a diagnostic evaluation to understand the cause of the transmission behavior.
Professional Technical Analysis
The 2004 Subaru Outback equipped with a 2.5L H4 engine and automatic transmission, likely has around 150,000 miles on it. The primary complaint involves an interlock issue indicated by the P1799 code, which typically relates to the transmission control system's failure to communicate properly with the engine control module (ECM). The simultaneous illumination of the Check Engine Light and the sport mode light suggests a fault that is preventing the transmission from shifting correctly. This could be due to several factors including a malfunctioning transmission control module (TCM), electrical connectors, or wiring issues. It's important to analyze the interconnection between the TCM and ECM, as any failure in this communication can lead to erratic transmission behavior. Given the age and mileage of the vehicle, wear and tear on electrical connections is a common issue, particularly in high-humidity environments. The current condition of the vehicle requires immediate attention to avoid further transmission damage and ensure safe operation.
Possible Causes
Most common causes (ordered by frequency):
- Transmission Control Module (TCM) Failure: The TCM may not be processing signals correctly, which is common in vehicles with high mileage. A faulty TCM can fail to engage the proper shift solenoids, leading to the transmission's inability to upshift. Testing voltage inputs and outputs from the TCM can reveal irregularities, with expected output voltages around 12V during operation. If the TCM is found defective, it should be replaced with an OEM part (P/N 31701AE11A) costing approximately $400-600. - Symptoms include failure to upshift, presence of P1799 code, and intermittent illumination of warning lights.
- Electrical Connector Issues: Corroded or loose connections between the TCM and ECM can disrupt communication, leading to failure in shifting. Visual inspection and continuity testing of connectors should be performed. The resistance should be below 5 ohms, and any signs of corrosion or damage should be addressed immediately. - Check for signs of corrosion at connectors, intermittent connection issues, and fluctuating voltage readings.
- Wiring Harness Damage: The wiring harness associated with the TCM could be damaged or shorted, causing signal loss. A thorough inspection of the wiring harness should be conducted. Resistance in the harness should be below 3 ohms, and any damaged wires should be repaired or replaced. - Visual inspection revealing frayed wires or signs of abrasion, along with inconsistent signal transmission during testing.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - OBD-II Code Retrieval: Utilize a professional OBD-II scanner or GeekOBD APP to retrieve the stored trouble codes. Ensure to examine any freeze frame data associated with P1799, which can provide additional context to the failure. Pay attention to any other pending codes as they may indicate related issues.
- STEP 2 - Visual Inspection: Conduct a thorough visual inspection of the wiring harness and connectors associated with the TCM and ECM. Look for any signs of wear, corrosion, or damage that could affect connectivity or signal transmission. Ensure that all connectors are seated properly and secure.
- STEP 3 - Electrical Testing: Using a multimeter, check the resistance of the wiring harness and connectors. Resistance should be below 5 ohms for connectors and below 3 ohms for wiring. Additionally, test the voltage outputs from the TCM while the vehicle is in different operating modes, ensuring they are within specifications (around 12V).
- STEP 4 - Road Test: After addressing any issues found during inspections and testing, perform a controlled road test to observe the transmission behavior. Monitor the Check Engine Light and sport mode light for any reoccurrence and check for proper shifting under various load conditions.
