Comprehensive diagnostic guide for OBD-II code P0749

"P0749 - Intermittent fault in Torque Converter Clutch (TCC) Pressure Control Solenoid A" (Solenoid A pressure control, intermittent)

What This Code Means

• P0749 is a powertrain/torque converter related diagnostic code indicating an intermittent fault in the pressure control solenoid A that controls the torque converter clutch hydraulic pressure. The Open Source code definition explicitly identifies this as an intermittent fault of the pressure control solenoid A, and Wikipedia's OBD-II sections describe powertrain codes as fault indicators monitored by the vehicle's control systems (DTCs, including transmission-related codes) [Open Source code definitions; Wikipedia: OBD-II - Diagnostic Trouble Codes; Wikipedia: OBD-II - Powertrain Codes]. In short: intermittent electrical/hydraulic control of the TCC solenoid is not behaving as expected, triggering P0749.

Symptom possibilities (what real drivers may notice)

• Transmission shifting feels harsh, late, or erratic, especially during gear changes or when engaging/disengaging the torque converter.
• Noticeable torque converter slip or a feel of "unlocked" or slipping during acceleration.
• Intermittent check engine/ transmission warning light; P0749 may appear alone or with related transmission codes.
• Occasional loss of power or reduced performance; sometimes accompanied by poor fuel economy.
• In some cases, the vehicle may seem to operate normally for a period, then exhibit intermittent shifting or torque converter behavior again.

Important Notes

Probable Causes

Because do not include NHTSA complaint data for P0749, the probability estimates below come from general field experience and typical failure patterns seen with TCC solenoids and related circuitry:

• 45% - Intermittent or failed Pressure Control Solenoid A itself or its circuit (solenoid coil failure, intermittent internal contact, or poor electrical connection)
• 25% - Wiring/connector or harness issues to the solenoid (corrosion, loose pins, damaged insulation, short to ground or power)
• 15% - Transmission fluid issues affecting hydraulic pressure (low fluid level, contaminated/old fluid, incorrect viscosity)
• 10% - Valve body or hydraulic control issues (debris, worn valve body passages, or other hydraulic faults affecting TCC operation)
• 5% - PCM/TCM software or calibration-related anomaly (rare, but can manifest as intermittent solenoid command issues)

Diagnostic Approach

Safety and prep

• Ensure vehicle is on a flat surface, transmission is at safe operating temperature, and you have the appropriate tools: OBD-II scan tool capable of live data, multimeter, transmission fluid dipstick, and, if needed, a wiring diagram for the solenoid circuit. If you must access the transmission, follow all shop safety procedures and support the vehicle properly.

1) Confirm the code and gather data

• Scan for P0749 and any related transmission/torque converter codes (e.g., P0740, P0741, P0730, P0750, etc.). Note freeze-frame data: transmission temperature, vehicle speed at the time of the fault, engine RPM, and commanded gear/shifts. This helps correlate a pattern (e.g., intermittent at certain temps or speeds) and whether other codes are present.
• Look for any ongoing drivability complaints: shift harshness, slipping, or torque converter lockup behavior.

2) Inspect transmission fluid and overall condition

• Check the transmission fluid level and condition (color, smell, contamination). Low or dirty fluid can affect hydraulic pressure and solenoid operation.
• Confirm you're using the correct spec fluid for the transmission and vehicle. If the fluid is old or degraded, a service may be indicated (note: some vehicles require a filter/valve body service alongside a fluid change).

3) Electrical and harness inspection (solenoid A circuit)

• Locate TCC pressure control solenoid A and its circuit. Check for obvious issues: damaged wiring, chafed insulation, exposed conductors, or signs of overheating.
• Inspect the solenoid connector for corrosion, bent pins, or moisture ingress. Reseat connectors firmly.
• Check system power and ground:
  • With the ignition on, you should have the proper supply voltage at the solenoid circuit (per the vehicle's service information). Measure the supply voltage and compare to specifications.
  • Verify a solid ground path for the solenoid circuit. Faulty ground can cause intermittent operation.
• If you have the ability to measure resistance, test the solenoid coil resistance at the connector pins when the solenoid is unplugged. Compare with the manufacturer's specifications for your exact transmission model. Out-of-spec resistance can indicate a defective solenoid.

4) Live data monitoring and functional testing

• Using a capable scan tool, monitor the solenoid A circuit in real time while performing a controlled driving test or a simulated load test:
  • Observe the solenoid duty cycle, commanded vs. actual pressure signals, or any fluctuating data related to the TCC pressure control circuit.
  • If the data shows intermittent loss of command or abnormal values when shifting or at engine load, this supports an intermittent solenoid/circuit fault.
• If the vehicle supports a diagnostic/functional test for the TCC solenoid, run it per the service information to observe whether the solenoid responds correctly.

5) Evaluate the hydraulic/valve-body side

• If electrical checks are within spec and the problem persists, suspect hydraulic control: valve body passages, debris, or a worn TCC/clutch hydraulic path.
• Depending on the vehicle, this may require a valve body service, solenoid replacement, or even a valve body replacement. This step often follows if the solenoid circuit tests pass but symptoms and data indicate hydraulic control is not applying correctly.

6) Consider mechanical transmission health

• In some cases, a worn clutch pack, internal wear, or input/output shaft issues can mimic or contribute to intermittent TCC control faults. This is typically a later-stage consideration if electrical/hydraulic diagnostics do not resolve the code or symptoms persist after a solenoid-related repair.

7) Repair options (order of likelihood and practicality)

• Electrical repair:
  • Clean, repair, or replace damaged wiring harness segments and connectors to the solenoid A.
  • Replace the TCC pressure control solenoid A if the coil/solenoid shows resistance out of spec, or the circuit tests indicate intermittent operation.
• Hydraulic/valve-body repair:
  • Replace or service the valve body if debris or worn passages are found that affect TCC hydraulic control.
• Fluid-related fix:
  • Replace transmission fluid and filter (if applicable) and recheck system. Ensure the correct fluid type and capacity.
• System calibration/updates:
  • If software or calibration issues are suspected (rare), update the TCM/PCM software to the latest revision per the vehicle-maker's guidance. Relearn or adapt the transmission after major repairs if required.
• After repair:
  • Clear P0749 and any related codes.
  • Perform a road test to verify no reoccurrence of P0749 and that transmission shifts are smooth and consistent.
  • Recheck live data to confirm the solenoid A circuit operates within normal ranges during driving.

Post-Repair Verification

• Ensure no residual fault codes are stored and that the vehicle's drivability is acceptable. Confirm that the TCC engages and disengages properly and that shifts are smooth.
• If symptoms persist after a single-component repair (e.g., only the solenoid A), re-evaluate for possible valve-body issues, wiring harness damage, or a need for additional transmission service.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

---