P0735

What Does Code P0735 Mean?

The OBD-II diagnostic trouble code P0735 signifies an “Incorrect Gear Ratio for Gear 5.” This code is set by the Transmission Control Module (TCM), or Powertrain Control Module (PCM) in some integrated systems, when it detects that the actual gear ratio of the transmission does not match the expected gear ratio for 5th gear. The TCM/PCM constantly monitors the rotational speed of the transmission’s input shaft (via the turbine speed sensor) and the output shaft (via the vehicle speed sensor). By comparing these two speeds, the module calculates the current operational gear ratio. When the transmission is commanded into 5th gear, the TCM/PCM expects a specific ratio between the input and output speeds, as defined by the manufacturer’s specifications. If the calculated ratio deviates from the acceptable range for 5th gear for a calibrated period or number of drive cycles, P0735 is stored, indicating a mechanical or hydraulic malfunction preventing proper 5th gear engagement or an electrical/sensor issue causing incorrect ratio interpretation. This typically affects the automatic transmission’s ability to operate efficiently in its highest overdrive gear.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be illuminated.
• Transmission slipping: Especially noticeable when the transmission attempts to engage or hold 5th gear.
• Inability to shift into 5th gear: The transmission may remain in 4th gear at highway speeds.
• Higher engine RPMs than normal: At highway speeds due to the vehicle operating in a lower gear.
• Reduced fuel economy: A direct result of the engine operating at higher RPMs.
• Transmission “limp mode” or “failsafe mode”: The TCM may default to a restricted gear range (e.g., 2nd or 3rd gear only) to prevent further damage.
• Harsh or delayed shifts: Particularly when attempting to engage or disengage 5th gear.

What Causes the Code P0735?

• Internal transmission mechanical failure: This is a primary cause, including worn or damaged 5th gear clutch packs, bands, planetary gear sets, or bearings within the transmission.
• Low or contaminated transmission fluid: Insufficient fluid pressure can prevent proper clutch engagement for 5th gear, or degraded fluid can cause friction material wear.
• Faulty 5th gear shift solenoid: A sticking, damaged, or electrically open/shorted shift solenoid responsible for directing fluid pressure to the 5th gear clutch pack.
• Blocked transmission fluid passages: Debris or sludge can obstruct valve body passages or filter screens, restricting hydraulic pressure to the 5th gear circuit.
• Defective turbine speed sensor (input speed sensor) or vehicle speed sensor (output speed sensor): Inaccurate readings from these sensors can lead the TCM/PCM to miscalculate the gear ratio.
• Internal valve body issues: Worn valve body bores, sticking valves, or debris affecting the hydraulic control circuit for 5th gear.
• Damaged wiring or poor electrical connections: Corrosion, chafing, or an open/short circuit in the wiring harness affecting the speed sensors or shift solenoids.
• TCM/PCM internal failure or software anomaly: While less common, an internal fault in the control module or outdated/corrupted software can lead to incorrect gear ratio detection.

How to Diagnose and Troubleshoot

Diagnosing P0735 requires a systematic approach, combining visual inspection, diagnostic scan tool analysis, and electrical testing:

1. Initial Verification & Data Capture: Connect an OBD-II scanner to confirm P0735 is present. Check for any other related transmission or speed sensor codes. Document freeze frame data, as this provides crucial information about engine speed, vehicle speed, and transmission temperature at the time the code was set. Clear the code and road test the vehicle under conditions where 5th gear would normally engage to see if the code immediately returns.
2. Transmission Fluid Inspection: With the vehicle on a level surface and the transmission at operating temperature (unless manufacturer specifies cold check), check the transmission fluid level and condition. Low fluid levels can cause pressure issues. Burnt fluid, excessive particulate matter, or an unusual color indicates significant internal wear or contamination.
3. Visual Inspection: Inspect the external transmission wiring harness for any signs of damage, chafing, corrosion, or loose connections, particularly around the input/output speed sensors and transmission solenoid connectors. Check for any external fluid leaks.
4. Scan Tool Live Data Analysis:
   • Monitor the input shaft speed (ISS) and output shaft speed (OSS) sensor readings in live data. As you accelerate and decelerate, these speeds should be smooth and proportionate. Pay close attention to their relationship when the transmission attempts to shift into 5th gear. If one sensor shows erratic or zero readings, it points to a faulty sensor or circuit.
   • Observe the transmission fluid temperature (TFT) sensor reading. Abnormally high temperatures can indicate excessive slipping or a cooling system issue.
   • Monitor transmission gear commanded vs. actual gear (if supported by the scanner). When 5th gear is commanded, observe if the actual gear deviates.
   • Monitor the state of the shift solenoids. Verify that the TCM is commanding the correct solenoid(s) for 5th gear engagement.
5. Electrical Diagnostics (DMM):
   • Speed Sensor Testing: If the input or output speed sensor is suspected, test its resistance (for passive magnetic sensors) or verify its signal output using a DMM capable of frequency measurement or an oscilloscope (for active Hall-effect sensors). Ensure proper voltage supply and ground at the sensor connector.
   • Shift Solenoid Testing: Access the 5th gear shift solenoid (often requires dropping the transmission pan and removing the valve body cover). Using a DMM, test the solenoid’s resistance. Compare this reading to factory specifications. An open circuit or a very low resistance (short) indicates a faulty solenoid. Also, check for proper voltage supply and ground continuity at the solenoid connector from the TCM.
   • Wiring Integrity Check: Using a DMM, perform continuity tests on the wiring between the speed sensors/shift solenoids and the TCM/PCM connector. Check for shorts to ground or power in the harness.
6. Hydraulic Pressure Test: If electrical and sensor checks pass, and internal mechanical issues are suspected, a specialized transmission pressure gauge can be used to test line pressure and individual clutch circuit pressures. This advanced test helps identify valve body issues or failing clutch packs. This often requires professional equipment and expertise.

Recommended Repairs and Solutions

The resolution for P0735 is highly dependent on the root cause identified during diagnosis:

• Transmission Fluid Service: If the fluid level is low, top it off with the manufacturer-specified fluid type. If the fluid is burnt, discolored, or contaminated, perform a complete transmission fluid and filter change. Be cautious with flushing severely damaged transmissions, as new fluid may dislodge debris and worsen problems.
• Replace Faulty Speed Sensors: If the input speed sensor, output speed sensor, or related wiring is confirmed to be defective, replace the component with an OEM or high-quality aftermarket equivalent. Ensure connections are clean and secure.
• Replace Faulty Shift Solenoid: If the 5th gear shift solenoid is diagnosed as faulty, replace it. This typically involves removing the transmission pan and accessing the valve body assembly. It’s often recommended to inspect other solenoids while the pan is off.
• Valve Body Service or Replacement: If testing indicates issues within the valve body (e.g., sticking valves, worn bores, or internal leaks), the valve body may need to be disassembled, cleaned, inspected, and potentially reconditioned, or replaced as an assembly.
• Internal Transmission Repair or Replacement: If diagnosis points to significant internal mechanical damage, such as worn clutch packs, planetary gears, or bands, a full transmission overhaul or replacement with a remanufactured or new unit will be necessary. This is generally the most extensive and costly repair.
• Wiring Harness Repair: Any damaged, corroded, or shorted wiring harnesses or connectors must be repaired or replaced to restore proper electrical communication.
• TCM/PCM Reprogramming or Replacement: In rare instances where the TCM/PCM itself is faulty or requires a software update to correct a known issue, it may need to be reprogrammed or replaced. This typically requires specialized diagnostic tools available at dealerships or advanced independent shops.

Important Mechanic’s Tip: After any transmission component replacement or fluid service, always clear the diagnostic trouble codes and perform a comprehensive road test. Monitor live data to ensure the transmission shifts correctly through all gears, especially 5th gear, and that the code does not return. Always follow manufacturer-specific procedures for transmission fluid level checks, fluid types, and any necessary relearn procedures after component replacement to ensure proper operation and longevity.