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 shifting and protect the transmission. A low circuit condition typically means the sensor resistance is too low (shorted) or the signal voltage is below the expected range, often due to a faulty sensor, wiring issues, or a short to ground.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission slipping
- Reduced fuel economy
- Transmission overheating
2. Most Common Causes
- Faulty transmission fluid temperature sensor (internal resistance too low)
- Short to ground in the sensor wiring
- Corroded or damaged sensor connector with shorted pins
- Excessive resistance in circuit connections
- Intermittent wiring harness fault near moving components (e.g., shift lever)
- Defective PCM or TCM (rare)
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the sensor and wiring for damage, corrosion, or loose connections.
Step 3: Measure sensor resistance with a multimeter; compare to manufacturer specs (typically 1-10 kΩ at room temperature).
Step 4: Check for short to ground by disconnecting sensor and measuring resistance between signal wire and ground.
Step 5: If sensor and wiring are okay, test TCM input/output signals.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the transmission may shift improperly or overheat, leading to internal damage. Short trips to a repair shop are acceptable if the transmission is not slipping.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for parts and labor, depending on whether the sensor or wiring needs replacement. Diagnostic fees may add $50-$100.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.