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DTC codes / P09xx · Transmission / P0938

P0938Hydraulic Oil Temperature Sensor Range/Performance

P0938 indicates that the transmission control module (TCM) has detected an out-of-range or erratic signal from the hydraulic oil temperature sensor, which monitors the temperature of the transmission fluid. The computer compares the sensor's voltage against expected values for the current operating conditions and flags the code when the reading falls outside the valid range or behaves unpredictably.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light
  • Often no symptoms besides the warning light, especially in early stages
  • Harsh or delayed transmission shifts as the TCM substitutes default temperature values
  • Transmission may enter limp mode, limiting vehicle speed and gear selection
  • Erratic or inconsistent shift quality that varies with driving conditions
  • In rare cases, the transmission may slip or fail to engage a gear

Common causes, most likely first

  1. Faulty hydraulic oil temperature sensor (internal short or open circuit)
  2. Damaged or corroded wiring harness or connector at the sensor
  3. Poor electrical connection at the TCM or sensor ground
  4. Contaminated or degraded transmission fluid affecting sensor readings
  5. Internal transmission mechanical issues causing abnormal fluid temperatures
  6. TCM software glitch or internal fault

How to diagnose and fix it

  1. Use an OBD-II scanner to retrieve the code and check for related transmission codes
  2. Inspect the sensor wiring and connector for damage, corrosion, or loose pins
  3. Measure the sensor's resistance and compare it to the manufacturer's temperature-resistance chart
  4. Check transmission fluid level and condition; replace fluid if contaminated or degraded
  5. Verify the sensor's signal voltage at the TCM connector while wiggling the harness to detect intermittent faults
  6. If the sensor is faulty, replace it and clear the code, then test-drive to confirm the repair
  7. If the code persists, diagnose the TCM and internal transmission wiring as per service information

Don't overlook the transmission fluid condition

  • A degraded or contaminated transmission fluid can cause the sensor to send inaccurate readings, triggering P0938 even when the sensor itself is healthy.
  • Intermittent wiring faults often set P0938 without any drivability symptoms, so a thorough wiggle test of the harness is essential before replacing parts.
  • Some vehicles use the same sensor for multiple functions, and a failure can also set related codes like P0710 or P0711, so always check for stored codes.
  • The TCM may default to a fixed temperature value when the sensor signal is lost, masking the fault and leading to delayed or harsh shifts that owners misattribute to a mechanical problem.
  • After replacing the sensor, the TCM may require a relearn or adaptation reset to restore proper shift quality, which is often overlooked.

Can I keep driving with P0938?

If the vehicle drives normally with only the warning light on, you can continue driving to a repair shop, but avoid heavy loads or aggressive driving. If the transmission enters limp mode or shifts harshly, limit driving to a short distance and seek professional diagnosis promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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