Comprehensive diagnostic guide for OBD-II code P0742

Context and sources

• P0742 is a powertrain OBD-II diagnostic trouble code. OBD-II powertrain codes cover engine and transmission controls monitored by the PCM/TCM, and the system monitors parameters to detect faults. The general framework is that the onboard diagnostics monitor transmission and engine controls and flag faults via DTCs like P0742 when a fault is detected in the torque converter clutch circuit (P0742 is grouped with torque converter related codes in the powertrain domain) (
• An open-source code definition (MIT repository) maps P0742 to a torque converter clutch (TCC) lock-up circuit issue, described as "Torque converter clutch lock-up circuit applied." This supports the focus of P0742 on the TCC lock-up circuit, though exact wording and fault mode can vary by vehicle/ECU .
• The content does not give vehicle-specific wiring diagrams or exact resistance/spec ranges for a TCC solenoid, so the diagnostic guide relies on standard field procedure logic (electrical checks, fluid condition, and mechanical/valve body considerations) in line with general OBD-II/TCC fault handling.

Code at a glance (P0742)

• Description (typical interpretation): Torque Converter Clutch (TCC) lock-up circuit fault. Many vehicles interpret P0742 as a fault in the TCC lock-up circuit, where the PCM/TCM detects an abnormal condition in the TCC solenoid circuit or its control path (solenoid, wiring, or related circuitry). Some sources describe related outcomes as the TCC being stuck on or unable to lock up, depending on the vehicle's implementation.
• System area: Transmission (torque converter clutch circuit).
• Likely failure mode: Electrical/solenoid circuit fault, or a fault in the hydraulic/valve body path that prevents proper lock-up activation, possibly accompanied by poor lock-up engagement, slipping, or abnormal duty-cycle signals.
• Common symptoms (user-reported): MIL on with P0742 present; reduced fuel economy; erratic transmission behavior at highway speeds; possible abnormal lock/unlock feel during shifts or steady cruising; in some cases, failure to lock up the torque converter when commanded.

Important Notes

• According to Wikipedia, DTCs are used to monitor various engine/transmission parameters and will set a MIL when a fault is detected in the monitored parameter or circuit (Diagnostic Trouble Codes, OBD-II sections). P0742 is a powertrain code, thus within the scope of transmission control monitoring (
• The open-source description specifically identifies P0742 as related to the torque converter clutch lock-up circuit being applied (i.e., the lock-up circuit in question). This framing supports focusing diagnostics on the TCC solenoid circuit and lock-up control path .

Symptom-oriented diagnostic approach
Typical symptoms you might see with a P0742-present vehicle:

• MIL illumination (P0742 stored and possibly one or more related codes such as P0740/P0741 in some vehicles).
• Noticeable transmission behavior around highway speeds: weak or absent lock-up; or lock-up that engages/disengages irregularly.
• Degraded fuel economy due to lack of consistent TCC lock-up.
• Possible shudder or harshness when the TCC attempts to lock/unlock (depending on the vehicle and the exact fault mode).
• On some vehicles, the transmission may shift normally but with the TCC circuit fault not allowing proper lock-up, leading to higher engine RPM at cruising speeds.

Common root-cause categories (with estimated likelihood)
Note: The following probability estimates are qualitative and, not NHTSA complaint data (no such data provided ). They reflect typical patterns seen with P0742 across various makes/models.

• Wiring/connector issues in the TCC solenoid circuit (including harness damage, corrosion, pin/ground issues): 30-50%
• Faulty TCC solenoid or valve body issue (solenoid intermittent failure, sticking solenoid, or hydraulic control path problems in the valve body): 15-30%
• Transmission fluid condition (low level, old/contaminated fluid, degraded hydraulic performance affecting TCC operation): 10-25%
• PCM/TCM software or calibration-related faults (requires reflash or updated TCM software; less common): 5-15%
• Mechanical wear or failure of the torque converter or clutch-pack (slipping or inability to lock due to wear): 5-15%
• Other related electrical faults (e.g., poor ground, is/was ambiguous data from the PCM, communication errors with the TCM): 5-10%

Diagnostic procedure (step-by-step)

1) Verify and contextualize

• Confirm P0742 is current and not historical; review freeze-frame data and any related codes (P0740, P0741, P0743, etc., if present).
• Note vehicle specifics: make/model/transmission type, engine size, age, fluid history, and prior transmission service. Use this to tailor your test plan.

2) Visual and safety checks

• Inspect the TCC solenoid connector and wiring harness at the transmission valve body for corrosion, broken insulation, chafing, loose pins, or water intrusion.
• Check grounds and battery/alternator health; a poor electrical environment can produce spurious TCC signals.
• Confirm transmission fluid level and condition (color, odor, particulate matter). Low or degraded fluid can affect hydraulic control.

3) Electrical: power, ground, and coil resistance

• With ignition ON (engine OFF), verify that the TCC solenoid circuit has a stable 12V supply when commanded by the TCM/PCM, and verify a solid ground path.
• Measure the TCC solenoid coil resistance across the solenoid terminals and compare to vehicle service data/specs. Look for open circuit, short to power, or abnormal resistance that would indicate a faulty coil.
• If the wiring and connector appear damaged, repair or replace the harness as needed before replacing the solenoid.

4) Electrical path verification under control

• Using an appropriate scan tool, verify the PCM/TCM commanded TCC operation. Check:
  • When the vehicle reaches the appropriate speed/temperature, does the TCC command to lock? Does it respond with a detected lock or is there a fault that prevents lock?
  • Monitor TCC duty cycle or PWM signal if the tool provides that data, and observe for irregular duty cycles or no signal.
• If commanded signal is present but the TCC does not lock, suspect solenoid or hydraulic side of the valve body.

5) Fluid and hydraulic assessment

• If electrical tests are inconclusive, consider hydraulic issues in the valve body:
  • A dirty or contaminated valve body can impede proper TCC operation.
  • A clogged TCC passage or worn TCC clutch hydraulics can cause delays or failure to lock.
• Transmission fluid service history is important. If the fluid is old or burnt, it can affect hydraulic performance. Plan for corrective action if indicated (fluid change, filter, or longer-term transmission service as per manufacturer guidelines).

6) Mechanical inspection (if electrical and hydraulic checks do not identify a fault)

• Worn clutch material or a failing torque converter can manifest as inability to achieve proper lock-up under operation, even if the solenoid and hydraulic path are correct. This is generally a less common root cause but must be considered for high-mileage units or known transmission wear.

7) Software/Calibration considerations

• Check for TCM software updates or recalls for your vehicle; reprogramming or calibration updates can fix lock-up logic issues that trigger P0742.
• If a software fault is suspected, perform the vehicle-specific reflash/update per the manufacturer service bulletin (SB/TSB) instructions.

8) Testing plan summary

• Electrical tests: 12V supply and ground to TCC solenoid; coil resistance; continuity/connector integrity.
• Functional tests: commanded TCC lock-up via scan tool; monitor TCC duty cycle and response; verify lock-up engagement at conditions when expected.
• Hydraulic tests: valve body inspection, TCC circuit passages, and fluid condition assessment.
• Fluid service: if indicated, perform a controlled transmission fluid change (and filter, if specified) per OEM guidelines.
• Mechanical assessment: evaluate for clutch wear or converter issues if electrical/hydraulic paths test normal but lock-up still fails.
• Software: review and apply any TCM software updates or calibrations.

Repair/repair-validation strategies

• Most common repair path: repair/replace faulty TCC solenoid or its wiring harness. If the solenoid tests out-of-spec but the harness is intact, solenoid replacement is indicated.
• If wiring is damaged or corroded beyond repair, replace the harness or repair the damaged segments, ensuring proper shielding and routing to prevent future wear.
• If the valve body or hydraulic path is suspected, a valve body service or replacement may be required, often accompanied by a transmission fluid change.
• If the fluid is old/contaminated, perform a controlled drain/fill with OEM-spec transmission fluid, and replace the filter as recommended.
• If PCM/TCM software is out of date or flagged by manufacturer, perform the approved reflash/update.
• In cases of mechanical wear (clutch packs, torque converter wear), discuss replacement of the affected components (often a full or major transmission service) with the vehicle owner.

Post-Repair Verification

• Clear DTCs after repair and perform a road test under normal operating conditions.
• Use scan tool data to confirm that:
  • The TCC is commanded and actually locks at the appropriate vehicle speeds and temperatures.
  • No new codes appear related to the TCC or transmission hydraulics.
• Confirm fuel economy expectations and check for any drivability concerns during cruising and acceleration.

Notes on statements and citations

• The diagnostic framework and the concept that DTCs exist to monitor engine/transmission parameters come from Wikipedia's OBD-II sections on Diagnostic Trouble Codes and Powertrain Codes. This provides the general context for P0742 as a powertrain code tied to a transmission component (torque converter clutch) (
• The specific code interpretation for P0742 as (Torque Converter Clutch Lock-Up Circuit Applied). This supports focusing diagnostic efforts on the TCC lock-up circuit, solenoid, and hydraulic path .
• Given the lack of vehicle-specific data , the guide emphasizes standard diagnostic steps (visual inspection, electrical checks, hydraulic/valve body considerations, fluid condition, and software updates) as consistent with general OBD-II powertrain diagnostic practice described in .

When to Escalate

• If P0742 persists after addressing the above categories, or if there are multiple transmission faults, consider a more extensive transmission service, possible valve body replacement, or a full transmission overhaul as indicated by OEM service procedures. Always verify with the vehicle's service information for maker-specific procedures and torque specs.

Safety Considerations

• Work safely around a live transmission; depressurization and proper supports are essential when performing valve body service or fluid changes.
• Use proper PPE, avoid hot surfaces, and follow vehicle-specific service procedures for battery, ignition, and transmission work.

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.

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