The Infiniti Q50, known for its performance and luxury, can sometimes experience transmission issues. One common problem is indicated by the P0720 diagnostic trouble code, often associated with a faulty Q50 Tcm (Transmission Control Module). While the code suggests an output speed sensor issue, the root cause frequently lies within the valve body and TCM itself. This comprehensive guide provides a detailed walkthrough of diagnosing and replacing the valve body and TCM on a 2014-2016 Infiniti Q50 equipped with the VQ37VHR engine and RE7R01A transmission. This guide is for informational purposes only, and should not be attempted by inexperienced individuals. Consult a professional mechanic if you are not comfortable working on your vehicle.
Understanding the P0720 Code and Q50 TCM Issues
The P0720 code signifies a problem with the output speed sensor circuit. However, a National Highway Traffic Safety Administration (NHTSA) investigation (https://static.nhtsa.gov/odi/tsbs/2019/MC-10168892-0001.pdf) indicates that the sensor itself is rarely the culprit. Instead, the issue often stems from the valve body and integrated TCM within the transmission. Symptoms accompanying the P0720 code may include:
- Inability to shift beyond third gear
- Engine speed limited to 2,000 RPM in first gear
- Inability to engage manual shift mode
Required Tools and Parts for Q50 TCM Replacement
Before beginning the repair, gather the necessary tools and parts:
- New, refurbished, or used valve body with integrated Q50 TCM (ensure compatibility with your specific model year)
- 10 quarts of Nissan Matic S transmission fluid (OEM fluid is crucial)
- New transmission drain bolt (Part Number: 31377-31X06)
- New transmission fill bolt (Part Number: 31361-1XJ0A)
- New oil pan gasket (Part Number: 31397-1XJ0A)
- ATF filler adapter (e.g., CTA Tools 7425)
- 5 mm hex bit (for fill bolt)
- Torque wrench (capable of reading as low as 5 ft-lbs)
- 19 mm socket (for drain bolt)
- 10 mm socket (for other bolts)
- Fluid transfer pump
- 4 jack stands and a jack (or a vehicle lift)
- A second person to assist (highly recommended)
- Factory Service Manual (FSM) – Factory Service Manuals – NICOclub
Step-by-Step Q50 Valve Body and TCM Replacement Procedure
Disclaimer: This guide provides a general overview. Always consult the FSM for detailed diagrams and specific instructions for your Q50 model year.
1. Preparation:
- Secure the vehicle on jack stands or a lift, ensuring it is level.
- Remove the rear engine cover.
2. Draining and Accessing the Transmission:
- Drain the existing transmission fluid. Expect approximately 7 quarts to drain initially, with more after removing the pan.
- Remove the exhaust mounting bracket.
- Disconnect and secure the heated oxygen sensor connectors.
- Remove the bracket from the transmission assembly.
- Remove the oil pan and gasket. Clean the pan and magnets.
3. Removing the Old Valve Body and Q50 TCM:
- Remove the snap ring from the joint connector (refer to the FSM for specific instructions).
- Push the joint connector inward.
- Disconnect the output speed sensor connector.
- Remove the bolts securing the control valve and TCM, noting the different bolt sizes and their locations (refer to the FSM).
- Carefully remove the control valve and TCM assembly.
- Disconnect the joint connector and TCM harness connector.
4. Installing the New Valve Body and Q50 TCM:
- Transfer necessary parts from the old valve body to the new one (e.g., solenoid cover, TCM harness, joint connector bracket).
- Apply ATF to the O-ring of the new joint connector. Never reuse the old joint connector.
- Carefully install the new valve body and TCM, ensuring proper alignment of the manual valve cutout and projection plate.
- Install and tighten the bolts in the correct sequence and to the specified torque (refer to the FSM).
5. Reassembling and Filling the Transmission:
- Install a new oil pan gasket and tighten the oil pan bolts to the specified torque.
- Fill the transmission with approximately 3 liters of ATF using the fill adapter and pump.
- Temporarily tighten the overflow plug.
- Lower the vehicle.
- Start the engine and let it run for 3 minutes, then stop the engine.
- Repeat the fluid filling process several times, following the procedure outlined in the FSM (TM-204 – TM-207). This involves draining and refilling to ensure the correct fluid level.
Conclusion
Replacing the valve body and Q50 TCM can resolve the P0720 code and restore proper transmission function. This procedure requires careful attention to detail and adherence to the FSM instructions. If you are unsure about any step, consult a qualified mechanic. Always use OEM parts and fluids for optimal performance and longevity. Driving with a faulty transmission can lead to further damage, so addressing this issue promptly is crucial.
