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P0698 — Sensor Reference Voltage C Circuit Low

Generic OBD-II · all makesPowertrain (engine & transmission)

P0698 indicates that the PCM has detected a low voltage condition on the Sensor Reference Voltage C circuit. This is a 5-volt reference line shared by one or more engine or transmission sensors, and a low reading means the reference signal has dropped below the expected threshold due to a short, excessive draw, or wiring fault.

Symptoms

What causes it — most likely first

1. Shorted sensor on the 5V reference C circuit

A sensor powered by this reference line may have an internal short to ground, pulling the reference voltage down. Disconnecting sensors one at a time can isolate the faulty unit.

2. Wiring short to ground or chafing

The reference circuit wiring may be rubbed through, pinched, or shorted to ground somewhere between the PCM and the affected sensor(s).

3. Corroded or loose connector terminals

Corrosion, spread pins, or poor terminal contact at a sensor or PCM connector can cause excessive resistance or an intermittent short on the reference line.

4. Open circuit or high resistance in reference wiring

A broken wire or high-resistance connection can prevent the reference voltage from reaching the sensor properly, causing a low reading at the PCM.

5. PCM internal reference regulator failure

Rarely, the PCM's internal 5-volt reference regulator can fail, causing low output on the reference C circuit specifically. This should only be considered after all wiring and sensors are verified.

How it's diagnosed

  1. Read and record all stored codes; note any sensor codes that share the 5V reference C circuit for context.
  2. Visually inspect wiring harnesses and connectors for chafing, melting, corrosion, or pinched wires along the reference C circuit.
  3. With key on and engine off, backprobe the 5V reference C wire at the PCM connector and at each affected sensor to confirm the low voltage condition.
  4. Disconnect sensors on the reference C circuit one at a time while monitoring reference voltage; if voltage returns to normal when a sensor is unplugged, that sensor is the cause.
  5. Perform continuity and short-to-ground tests on the reference C wiring between the PCM and each sensor connector.
  6. If all sensors and wiring test normal, verify PCM power and ground supplies before considering PCM replacement.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationSAE generic — identical on every make
Related codesP0690 — Ecmpcm Power Relay Sense Circuit HighP0691 — Fan 1 Control Circuit LowP0692 — Fan 1 Control Circuit HighP0693 — Fan 2 Control Circuit LowP0694 — Fan 2 Control Circuit HighP0695 — Fan 3 Control Circuit Low

Frequently asked

Does P0698 mean I need to replace the PCM?

No. The PCM is the least likely cause. A shorted sensor, chafed wiring, or corroded connector pulls the 5-volt reference down far more often than an internal PCM failure. Replacing the PCM without ruling out sensors and wiring is a common and costly mistake.

Can I still drive the vehicle with this code?

It depends on symptoms. If the engine runs normally, short-distance driving may be possible, but sensors relying on the reference voltage may report inaccurate data, affecting drivability and emissions. Diagnosis should not be delayed.

Why are there other sensor codes stored along with P0698?

Because the 5V reference C circuit is shared by multiple sensors, a short or voltage drop on that line can cause every sensor on it to set its own code. Fixing the reference circuit issue often clears the related sensor codes.

How do I find which sensor is shorting the reference line?

The most practical method is to unplug sensors on the reference C circuit one at a time while watching the reference voltage. When the voltage returns to approximately 5 volts after disconnecting a particular sensor, that sensor is likely the culprit.