Sensor Reference Voltage "C" Circuit Low
P0698 indicates that the engine control module has detected low voltage in the sensor reference voltage C circuit. This circuit provides a stable 5-volt reference signal to various sensors, and when voltage drops below normal operating levels, the ECM cannot properly read sensor data. This typically affects transmission shift logic and sensor accuracy across multiple systems.
What You Might Notice
- Check engine light illuminated
- Transmission fails to shift between sport and economy modes
- Erratic or delayed transmission shifting
- Reduced fuel economy
- Poor engine performance or hesitation
Most Common Causes
- 1
Faulty Sensor Reference Voltage Relay
The power relay supplying the reference voltage circuit may be failing or sticking, preventing proper voltage delivery to the reference circuit.
- 2
Blown Fuse or Fusible Link
A blown fuse or fusible link in the sensor reference voltage circuit will cause voltage to drop below the required 5V threshold.
- 3
Open or Corroded Connector/Wiring
Corroded connectors, broken wires, or loose connections in the sensor reference voltage circuit create resistance that drops voltage below specification.
- 4
Defective Engine Control Module
A failing ECM may lose its ability to regulate or supply proper reference voltage to the sensor circuit, though this is less common than electrical faults.
How to Diagnose It
- 1
Check Faulty Sensor Reference Voltage Relay — The power relay supplying the reference voltage circuit may be failing or sticking, preventing proper voltage delivery to the reference circuit.
- 2
Check Blown Fuse or Fusible Link — A blown fuse or fusible link in the sensor reference voltage circuit will cause voltage to drop below the required 5V threshold.
- 3
Check Open or Corroded Connector/Wiring — Corroded connectors, broken wires, or loose connections in the sensor reference voltage circuit create resistance that drops voltage below specification.
- 4
Check Defective Engine Control Module — A failing ECM may lose its ability to regulate or supply proper reference voltage to the sensor circuit, though this is less common than electrical faults.
How to Fix It
- 1
Inspect and Replace Sensor Reference Relay
Locate the sensor reference voltage relay in the fuse/relay box, test it with a multimeter for proper operation, and replace if faulty. Verify proper voltage output after replacement.
- 2
Check and Replace Blown Fuses
Inspect the fuse box for any blown fuses related to sensor power circuits. Replace with the correct amperage rating and retest for voltage.
- 3
Clean and Repair Connectors and Wiring
Disconnect and visually inspect all connectors in the sensor reference voltage circuit for corrosion, loose pins, or damage. Clean with electrical cleaner, repair loose connections, and replace damaged wiring as needed.
- 4
Test and Replace ECM if Necessary
If voltage supply and wiring test normal, use specialized diagnostic equipment to test ECM output. If the ECM cannot maintain proper reference voltage, it may require replacement or reprogramming.
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