Transmission Fluid Pressure Sensor/Switch Circuit Intermittent
The P0991 code indicates an intermittent electrical issue in the transmission fluid pressure sensor or switch circuit. This sensor monitors hydraulic pressure inside the transmission to ensure proper shift timing and line pressure. An intermittent signal can cause erratic shifting, slipping, or limp mode. Common causes include a faulty sensor, damaged wiring, poor electrical connections, or low transmission fluid level.
1. Symptoms You Will Notice
- Erratic or harsh shifting
- Transmission slipping or delayed engagement
- Check Engine Light illuminated
- Transmission stuck in limp mode
2. Most Common Causes
- Faulty transmission fluid pressure sensor
- Damaged or corroded wiring/connectors
- Intermittent open or short in circuit
- Low or contaminated transmission fluid
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect transmission fluid level and condition.
Step 3: Visually check wiring and connectors for damage or corrosion.
Step 4: Test sensor resistance and compare to manufacturer specs.
Step 5: Check for intermittent shorts by wiggling harness while monitoring sensor signal.
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 erratically or fail, potentially causing further damage.
How much does it cost to fix?▼
Repair costs typically range from $50 to $150 for sensor replacement, plus diagnostic time. Wiring repairs may add to the cost.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.