Hydraulic Oil Temperature Sensor Circuit Low
The P0939 code indicates that the transmission control module (TCM) has detected a low voltage signal from the hydraulic oil temperature sensor circuit. This sensor monitors the temperature of the transmission fluid to optimize shift schedules and torque converter lockup. A low circuit condition typically means the sensor resistance is too low (indicating a short to ground) or the signal voltage is below the expected range. This can lead to incorrect temperature readings, causing the TCM to assume the fluid is cold and potentially delay shifts or prevent torque converter lockup.
01. Most Common Causes
- Faulty hydraulic oil temperature sensor
- Short to ground in the sensor circuit wiring
- Damaged connector or corroded terminals
- Internal transmission harness issue
- Faulty TCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifts
- Torque converter not locking up
- Transmission stuck in limp mode
- Poor fuel economy
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect the sensor connector and wiring for damage, corrosion, or shorts.
Step 3: Measure sensor resistance at the connector; compare to manufacturer specs (typically 1-10 kΩ at room temperature).
Step 4: Check for voltage at the sensor signal wire (should be ~5V reference with ignition on).
Step 5: If wiring and sensor are good, test TCM output or replace TCM as last resort.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances. The transmission may shift poorly or enter limp mode, which can cause further damage. Have it diagnosed promptly.
How much does it cost to fix?▼
Typically $50-$150 for a sensor replacement if DIY, or $150-$300 at a shop including labor and diagnostics.