P0685 - Engine Controls Ignition Relay Control Circuit (PCM)

Introduction:

The P0685 diagnostic trouble code (DTC) indicates a problem within the engine control system related to the ignition relay control circuit. This circuit is crucial for providing power to various engine components, allowing the vehicle to start and run. A malfunctioning ignition relay control circuit can lead to a no-start condition, stalling, and other drivability issues. Understanding the causes, symptoms, and troubleshooting steps associated with P0685 is essential for accurate diagnosis and repair.

Comprehensive Table: P0685 - Engine Controls Ignition Relay Control Circuit (PCM)

Category	Description	Possible Causes
Definition	Engine Controls Ignition Relay Control Circuit (PCM) - Indicates a fault in the circuit that the PCM uses to control the ignition relay.
Symptoms	Common signs that the P0685 code is present.	No-start condition, Engine stalls intermittently, Check Engine Light (CEL) illumination, Cranking but no ignition, Reduced engine power, Starting problems especially after engine is warm, Battery drain (in rare cases).
Possible Causes	The root problems that could trigger this code.	Faulty ignition relay, Open or shorted wiring in the ignition relay control circuit, Corroded or loose connections in the ignition relay control circuit, Defective PCM (Powertrain Control Module), Faulty Crankshaft Position Sensor, Faulty Camshaft Position Sensor, Low battery voltage, Failing ignition switch, Blown fuse(s) related to the ignition system, Grounding issues.
Diagnostic Tools	Equipment needed for diagnosing the problem.	Multimeter, Scan tool (OBD-II scanner), Wiring diagram for the vehicle's ignition system, Test light, Circuit tester.
Troubleshooting Steps	Steps to identify and isolate the problem.	Visual inspection of wiring and connectors, Testing the ignition relay, Checking for voltage at the ignition relay, Testing the PCM's control signal to the ignition relay, Checking fuses and grounds, Inspecting the crankshaft and camshaft position sensors, Performing a PCM reset, Checking battery voltage.
Common Misdiagnoses	Errors that are often made when diagnosing P0685.	Replacing the PCM without proper diagnosis, Replacing the ignition relay without checking the wiring, Ignoring grounding issues, Overlooking other related codes.
Related Codes	Other codes that may appear alongside P0685.	P0686 (Ignition Relay Control Circuit Low), P0687 (Ignition Relay Control Circuit High), P0688 (Ignition Relay Sense Circuit Open), P0689 (Ignition Relay Sense Circuit Low), P0690 (Ignition Relay Sense Circuit High), P0335 (Crankshaft Position Sensor "A" Circuit), P0340 (Camshaft Position Sensor "A" Circuit).
Repair Costs (Estimate)	Approximate cost of repairs, parts and labor.	Ignition Relay: $20 - $100, Wiring Repair: $50 - $300, PCM Replacement: $500 - $1500 (including programming), Labor costs vary widely.
Technical Service Bulletins	Manufacturer-issued notices that address common problems.	Check for any TSBs related to the P0685 code and the specific vehicle make and model.
Impact on Vehicle Operation	Effects of the P0685 code on the vehicle's functionality.	Prevents the engine from starting, can cause stalling, may lead to reduced engine performance.
Importance of Proper Repair	Consequences of ignoring or improperly repairing the P0685 code.	Potential for further damage to the engine or electrical system, safety concerns due to unexpected stalling, failure to pass emissions tests.
Testing the Ignition Relay	A detailed explanation of how to check the ignition relay.	Use a multimeter to check for continuity across the relay's coil. Apply power to the coil and check for continuity across the relay's switched contacts. A test light can also be used to verify power is flowing through the relay when activated.
PCM Role	The PCM's function in the ignition relay control circuit.	The PCM provides the ground signal necessary to activate the ignition relay. It monitors the ignition system and controls the relay based on various sensor inputs. A faulty PCM may fail to send the correct signal or misinterpret sensor data.

Detailed Explanations:

Definition: The P0685 code indicates that the Powertrain Control Module (PCM) has detected a malfunction in the circuit it uses to control the ignition relay. The PCM monitors the voltage and current in this circuit, and if it detects an abnormal reading, it sets the P0685 code.

Symptoms: Common symptoms include a no-start condition (the engine cranks but doesn't start), intermittent stalling, the Check Engine Light (CEL) illuminating, the engine cranking but failing to ignite, reduced engine power, difficulty starting the engine, especially when warm, and, in rare cases, battery drain.

Possible Causes: The most frequent culprits are a faulty ignition relay, open or shorted wiring within the ignition relay control circuit, corroded or loose connections in the same circuit, a defective PCM (though this is less common), faulty crankshaft or camshaft position sensors, low battery voltage, a failing ignition switch, blown fuses related to the ignition system, and grounding issues.

Diagnostic Tools: Essential tools include a multimeter for testing voltage and continuity, a scan tool (OBD-II scanner) for reading DTCs and live data, a wiring diagram specific to the vehicle's ignition system, a test light for checking power, and a circuit tester for more comprehensive circuit analysis.

Troubleshooting Steps: Begin with a thorough visual inspection of the wiring and connectors associated with the ignition relay. Test the ignition relay itself using a multimeter. Check for proper voltage at the ignition relay. Test the PCM's control signal to the ignition relay. Examine fuses and grounds related to the ignition system. Inspect the crankshaft and camshaft position sensors. Consider performing a PCM reset. Finally, verify that the battery voltage is within the acceptable range.

Common Misdiagnoses: A frequent mistake is replacing the PCM without performing thorough diagnostics. Replacing the ignition relay without inspecting the wiring is another common error. Ignoring grounding issues or overlooking other related diagnostic trouble codes can also lead to misdiagnosis.

Related Codes: Codes that might accompany P0685 include P0686 (Ignition Relay Control Circuit Low), P0687 (Ignition Relay Control Circuit High), P0688 (Ignition Relay Sense Circuit Open), P0689 (Ignition Relay Sense Circuit Low), P0690 (Ignition Relay Sense Circuit High), P0335 (Crankshaft Position Sensor "A" Circuit), and P0340 (Camshaft Position Sensor "A" Circuit). These related codes can provide further clues to the underlying problem.

Repair Costs (Estimate): The cost of repair can vary significantly. An ignition relay typically costs between $20 and $100. Wiring repairs can range from $50 to $300. PCM replacement, including programming, can be a more significant expense, ranging from $500 to $1500. Labor costs will vary depending on the mechanic and the complexity of the repair.

Technical Service Bulletins: Always check for any Technical Service Bulletins (TSBs) issued by the vehicle manufacturer that pertain to the P0685 code and the specific make and model of the vehicle. TSBs often contain valuable information about known issues and recommended repair procedures.

Impact on Vehicle Operation: The P0685 code can prevent the engine from starting, cause intermittent stalling, and potentially lead to reduced engine performance. The severity of the impact depends on the nature of the underlying problem.

Importance of Proper Repair: Ignoring or improperly repairing the P0685 code can lead to further damage to the engine or electrical system. It can also create safety concerns due to unexpected stalling and may result in the vehicle failing emissions tests.

Testing the Ignition Relay: Testing the ignition relay is crucial for determining if it's the source of the problem. Use a multimeter to check for continuity across the relay's coil. Then, apply power to the coil (typically 12V) and check for continuity across the relay's switched contacts. A test light can also be used to verify that power is flowing through the relay when it is activated.

PCM Role: The PCM plays a critical role in the ignition relay control circuit. It provides the ground signal necessary to activate the ignition relay. The PCM monitors the ignition system and controls the relay based on input from various sensors. A faulty PCM may fail to send the correct signal or misinterpret sensor data, leading to the P0685 code.

Frequently Asked Questions:

What does the P0685 code mean? It signifies a problem in the ignition relay control circuit, specifically the circuit controlled by the PCM.
Can I drive with a P0685 code? It is not recommended, as it can lead to stalling or a no-start condition, potentially causing safety issues.
Is the P0685 code a serious problem? Yes, it can prevent the engine from starting and should be addressed promptly to avoid further complications.
How do I fix a P0685 code? Troubleshooting involves checking the ignition relay, wiring, PCM, and related components, then repairing or replacing the faulty part.
Will replacing the ignition relay always fix the P0685 code? Not necessarily. The problem could also be with the wiring, PCM, or other components in the circuit.

Conclusion:

The P0685 code indicates a fault within the engine's ignition relay control circuit, often resulting in starting and drivability issues. Thorough diagnosis, using the appropriate tools and steps, is crucial to pinpoint the root cause and implement the correct repair, ensuring reliable vehicle operation.