P0698 Code: Understanding and Troubleshooting the 5V Reference 3 Circuit Low Voltage

The Diagnostic Trouble Code (DTC) P0698 indicates a problem within your vehicle’s 5-Volt Reference 3 Circuit. This circuit is crucial for providing a stable voltage supply to various sensors, which your car’s Engine Control Module (ECM) relies on for accurate readings and optimal engine performance. When the ECM detects that the voltage in the 5-Volt Reference 3 Circuit has dropped below the expected threshold, it triggers the P0698 code, signaling a potential issue that needs attention.

Understanding the P0698 code is the first step in effectively diagnosing and resolving the underlying problem. This article will delve into the specifics of DTC P0698, exploring its symptoms, potential causes, and providing a step-by-step guide to diagnosing and fixing the issue.

What Does DTC P0698 Mean?

DTC P0698 is formally defined as “5-Volt Reference 3 Circuit Low Voltage”. In simpler terms, it means the ECM has detected that the voltage in the 5-Volt Reference 3 Circuit is lower than expected – specifically, below 4.7 volts for a duration of 2 seconds or more.

To grasp the significance of this, it’s important to understand the role of 5-volt reference circuits in modern vehicles. Many sensors in your car, such as the Accelerator Pedal Position (APP) sensor and others, operate on a 5-volt reference signal provided by the ECM. This stable 5-volt supply allows these sensors to send accurate and consistent data back to the ECM, which is essential for proper engine control, fuel efficiency, and overall vehicle operation.

The “Reference 3 Circuit” designation implies that there might be multiple 5-volt reference circuits in your vehicle (Reference 1, Reference 2, etc.). While these circuits are independent outside the ECM, they are internally connected within the ECM. This means a problem in one sensor’s 5-volt reference circuit can potentially affect other sensors sharing the same internal ECM bussing.

Symptoms of a P0698 Code

When the P0698 code is triggered, you may observe several symptoms in your vehicle, including:

• Check Engine Light: The most immediate and obvious symptom is the illumination of the Malfunction Indicator Lamp (MIL), commonly known as the check engine light.
• Reduced Engine Performance: The ECM relies on accurate sensor data for optimal engine management. A low voltage in the reference circuit can lead to inaccurate sensor readings, potentially causing reduced engine power, hesitation, or rough idling.
• Transmission Issues: In some cases, issues with sensor reference voltages can also affect transmission performance, leading to erratic shifting or other transmission-related problems.
• ABS or Traction Control Light: Depending on the vehicle and the sensors affected, other warning lights like the ABS or traction control light might also illuminate.
• Other DTCs: P0698 may appear alongside other DTCs related to sensors that rely on the 5-Volt Reference 3 Circuit.

It’s important to note that these symptoms can vary depending on the specific vehicle, the severity of the voltage drop, and the sensors affected by the P0698 code.

Potential Causes of P0698

Several factors can contribute to a low voltage condition in the 5-Volt Reference 3 Circuit, leading to a P0698 code:

• Faulty Sensor: A malfunctioning sensor connected to the 5-Volt Reference 3 Circuit can draw excessive current, causing a voltage drop in the circuit. The Accelerator Pedal Position (APP) sensor is a common culprit, but other sensors on this circuit could also be the issue.
• Short Circuit to Ground: A short circuit in the 5-Volt Reference 3 Circuit wiring, where the wire is making unintended contact with the vehicle’s ground, can drain voltage and trigger the P0698 code.
• Wiring Issues: Damaged, corroded, or chafed wiring in the 5-Volt Reference 3 Circuit can lead to resistance and voltage drop.
• ECM Malfunction (Less Likely): In rare cases, a fault within the ECM itself could be the cause of the low reference voltage. However, this is usually considered a last resort after ruling out other potential causes.

Diagnosing and Troubleshooting P0698

Diagnosing a P0698 code requires a systematic approach to pinpoint the root cause of the low voltage in the 5-Volt Reference 3 Circuit. Here’s a step-by-step diagnostic procedure:

1. Initial Checks and Preparation:

• Diagnostic System Check: Before proceeding, perform a general Diagnostic System Check of the vehicle to ensure there are no other underlying issues that might be contributing to the P0698 code.
• Review Diagnostic Information: Familiarize yourself with the DTC descriptors, conditions for running and setting the DTC, and circuit/system descriptions as provided in the original article.
• Gather Tools: You will need a Digital Multimeter (DMM) to measure voltage and potentially wiring diagrams for your specific vehicle to identify components connected to the 5-Volt Reference 3 Circuit.

2. Circuit/System Verification:

• Engine Start and Run Check: Verify if the engine starts and runs. While the original article mentions this under “Circuit/System Verification,” it’s more of a general observation rather than a specific verification step for P0698. A running engine doesn’t rule out P0698, but it helps to know the overall engine condition.

3. Circuit/System Testing:

• Isolate the APP Sensor:

  1. Turn OFF the ignition and wait for the ECM to power down (approximately 60 seconds).
  2. Disconnect the Accelerator Pedal Position (APP) sensor. This is a common component on the 5-Volt Reference 3 Circuit and a frequent cause of P0698.
  3. Turn ON the ignition, with the engine OFF.
  4. Using your DMM, measure the voltage between the 5-volt reference circuit pin at the APP sensor connector and a good ground.
  5. Expected Result: The voltage should measure between 4.7 and 5.2 volts.
  6. If Voltage is Correct (4.7-5.2V): This indicates the 5-volt reference circuit from the ECM is likely functioning correctly up to the APP sensor connector. The problem might be a faulty APP sensor itself. Proceed to replace the APP sensor and retest.
  7. If Voltage is Still Low (Below 4.7V): This suggests the issue is not solely the APP sensor, or there might be another problem further upstream in the circuit. Proceed to the next step.

• Isolate Other Components on the 5-Volt Reference 3 Circuit:

  1. With the DMM still connected to the 5-volt reference circuit (at the APP sensor connector or another accessible point on the 5-Volt Reference 3 Circuit), systematically disconnect other components one by one that are connected to this circuit. Refer to wiring diagrams for your vehicle to identify these components. Common sensors on 5V reference circuits can include MAP sensors, TPS sensors, and others.
  2. Monitor Voltage: After disconnecting each component, observe if the voltage reading on your DMM changes.
  3. Identify Faulty Component: If the voltage reading jumps back up to the normal range (4.7-5.2V) when a specific component is disconnected, that component is likely the source of the problem. Replace the suspected faulty component and retest.

• Check for Short to Ground or Voltage:

  1. If isolating components doesn’t resolve the low voltage issue, the next step is to test the 5-Volt Reference 3 Circuit wiring itself for shorts.
  2. Short to Ground Test: With the ignition OFF and components disconnected from the 5-Volt Reference 3 Circuit, use your DMM to check for continuity between the 5-volt reference circuit wire and a good ground. There should be no continuity (infinite resistance). Continuity indicates a short to ground.
  3. Short to Voltage Test: Check for shorts to voltage, although this is less likely to cause a “low voltage” code. You would typically look for voltage on the 5-volt reference circuit wire with the ignition OFF (which should not be present).
  4. Repair Wiring: If a short to ground or other wiring issue is found, carefully inspect the wiring harness for damage, chafing, or corrosion. Repair any damaged wiring and retest.

• ECM Replacement (Last Resort):

  1. If all other tests have been performed and the P0698 code persists, and you have ruled out wiring and sensor issues, then the ECM itself may be faulty.
  2. Consider ECM Testing/Replacement: ECM replacement should be considered as a last resort and might require professional programming or calibration after replacement.

4. Post-Repair Verification:

• Clear DTCs: After completing repairs, clear the P0698 DTC using a scan tool.
• Verify Repair: Operate the vehicle and monitor for the reappearance of the P0698 code. Perform a drive cycle if necessary to ensure the issue is resolved.

By following these diagnostic steps, you can systematically troubleshoot the P0698 code and restore the proper function of your vehicle’s 5-Volt Reference 3 Circuit, ensuring optimal engine performance and sensor accuracy. Remember to always consult your vehicle’s service manual for specific wiring diagrams and component locations relevant to your make and model.