Comprehensive diagnostic guide for OBD-II code P0689 ECM/PCM Power Relay De-energized

Important Notes

• The available technical references from Wikipedia describe OBD-II concepts, including Diagnostic Trouble Codes, Powertrain Codes, and Emissions Testing, and provide context for how DTCs fit into modern engine management and emissions controls. These sources help define what powertrain DTCs are and how the OBD-II system generally operates (without vehicle-specific wiring diagrams).
• An Open Source entry labeled (Low signal from ECM/PCM power relay sensor) is listed, suggesting a focus on the ECM/PCM power-relay circuit. This hints at the general interpretation of P0689 as a fault in the ECM/PCM power relay circuit, though the entry itself is not vehicle-specific or mapped to a particular OEM code.

What This Code Means

• P0689 is categorized as an ECM/PCM power-relay circuit fault. In general terms, this code indicates that the Power Relay that supplies the ECM/PCM with operating power is not energizing as expected. When the ECM/PCM loses power, engine control and many essential systems may not function, often resulting in a no-start condition, stalling, or an immediate loss of control over certain engine functions.
• In practice, P0689 often relates to the relay itself, the control circuit that commands the relay, the main power feed to the ECM/PCM (fuses/fusible links), or the ECM/PCM itself (internal faults or software/logic issues) rather than a sensor in the traditional sense.

Symptoms

• Engine does not start or cranks but does not run, often with the engine no longer cranking or starting when the fault is intermittent.
• Check Engine Light (CEL) is illuminated with P0689 as a stored or pending DTC.
• Intermittent or permanent stalling while driving, followed by failure to restart until the relay/power feed is addressed.
• Accessories (if powered through the ECM/PCM or via the same supply path) may behave unpredictably or lose power momentarily.
• On some vehicles, you may observe voltage drop or warnings related to the battery/charging system because the ECM/PCM power feed is not being maintained.

Probable Causes

Note: P0689 is heavily tied to the ECM/PCM power-feed path. The following causes are listed in approximate order of likelihood in typical vehicles, with rough probability ranges:

• Faulty ECM/PCM power relay (or its coil/contacts) - 25% to 40%
• Wiring harness fault or poor connections in the relay circuit (including harness-to-relay, relay-to-ECM/PCM power pin, or grounds) - 25%
• Blown fuse or fusible link in the ECM/PCM power supply path - 10% to 15%
• Faulty ignition switch or ignition feed that fails to energize the relay control circuit - 10%
• Battery/charging issues causing insufficient voltage to energize the relay or maintain ECM/PCM power during cranking - 5%
• ECM/PCM internal fault or software issue affecting the relay control signal (less common, but possible) - 5%
• Ground integrity issues at the ECM/PCM or related circuits (lower-probability but possible) - 5%
  These percentages are intended as a practical guide for planning diagnosis and are not a guaranteed distribution for every vehicle. Actual results will vary by make/model and maintenance history.

Safety Considerations

• Disconnect power only after necessary, and ensure the vehicle is secured to prevent movement. When performing power-feed work, avoid shorting power and ground. Follow standard shop safety practices for electrical testing.
• If you must perform bench tests on the ECM/PCM or relays, use proper PPE and isolate the unit from battery sources to prevent damage or shock.

Diagnostic Approach

1) Verify the DTC and gather context

• Confirm P0689 is current or pending and check for related codes that might indicate additional faults (e.g., other power-related, ignition, or sensor codes).
• Check vehicle history and related symptoms (starting behavior, stalling, battery health, recent electrical work).

2) Basic power availability checks

• Test battery condition and charging system to ensure stable voltage (typical operating voltage > 12.6V with engine off; during cranking, voltage should remain above a functional threshold defined by the OEM, often above ~9-10V). If the battery or alternator is weak, it can affect the relay operation and PCM power circuit.
• Inspect and test fuses and fusible links associated with the ECM/PCM power supply. Replace any blown fuses as a suspected cause, but also investigate why the fuse blew (short, overload, or wiring issue).

3) Visual and mechanical inspection

• Inspect the ECM/PCM power relay for signs of overheating, pitting, or corrosion on coil and contact pins.
• Inspect relay socket, connectors, and wiring harness for looseness, bent pins, corrosion, or damaged insulation.
• Check ground connections for the ECM/PCM and the relay circuit; a high-resistance or open ground can cause strange PCM power behavior.

4) Relay control circuit testing (ECM/PCM side)

• With the ignition ON (engine OFF) or in a controlled testing environment, measure the coil control signal at the relay (the PCM/ECM output that energizes the relay). A proper PCM control signal should energize the relay coil and cause power to be delivered to the ECM/PCM.
• If the relay coil does not energize, diagnose the control circuit:
  • Check the input to the PCM that drives the relay (ignition feed, ignition switch supply, BCM control if applicable).
  • Look for blown fuses in the ignition/ignition-switch circuit or a fault in the ignition switch itself.

5) Relay performance testing (relay contacts and coil)

• Bench-test the suspected relay: measure coil resistance and energize with a known-good 12V source to confirm the coil pulls in. With the coil energized, measure contact continuity to verify the contacts close properly. If the coil does not pull in or contacts are fused/high resistance, replace the relay.
• If possible, swap with a known-good relay to see if the condition resolves. This is a quick, low-risk diagnostic step.

6) ECM/PCM power-feed path checks

• Measure voltage at the ECM/PCM power input pin(s) during an attempted energize (engine cranking or key-on). Verify there is voltage present when the PCM is expected to be powered.
• Check all intermediate connectors in the power-feed path (relay-to-ECM harness connectors, main power ground to the ECM, and the ignition-switched feed that supplies the relay coil).
• Look for excessive voltage drop along the power path. A significant drop indicates wiring resistance or poor connections that prevent the ECM/PCM from receiving stable power.

7) Determine if the issue is relay/control circuit vs PCM

• If the relay energizes and ECM receives power, but the PCM fails to monitor or send a proper control signal, or if the relay independently drops power during cranking, you may be facing an ECM/PCM internal fault or a latent control issue within the PCM.
• If, despite a good control signal, the relay coil fails to energize or power is not delivered to the ECM, suspect the relay, wiring, fuses, or ignition switch path.
• If ECM power is present but the PCM still cannot operate, consider PCM software or internal faults as a last resort after ruling out external causes.

8) Consider vehicle-specific variations

• Some vehicles may use multiple power circuits or additional relays for the ECM/PCM; one should verify there are not multiple fused or switched feeds (e.g., separate main power, ignition-switched power, and memory power) and ensure each feed remains healthy.

9) Summary of actionable checks (condensed)

• Battery and charging system: test and confirm stable voltage.
• Fuses and fusible links: inspect and replace as needed.
• ECM/PCM power relay: inspect, bench-test, and replace if necessary; consider swapping with a known-good unit.
• Wiring and connectors: inspect for corrosion, damage, and secure connections; verify ground integrity.
• Relay control circuit: verify PCM/ECM control signal is present when the ignition is on and during cranking.
• ECM/PCM power pin voltage: verify presence of proper voltage during energize attempts.
• If all exterior wiring and relay tests pass, consider ECM/PCM internal fault as a last resort, with OEM guidelines and possible reprogramming or replacement per manufacturer.

What to capture during diagnosis (documentation)

• Exact DTC listing (P0689) with any related codes.
• Battery voltage levels in key-on, crank, and running conditions.
• Fusing/fusible links status and any observed abnormalities.
• Relay part number, coil resistance, and contact condition.
• Relay control signal presence, timing, and voltage level at the ECM/PCM power pin(s).
• Condition of ECM/PCM power ground and harness connectors.
• Any environmental or mechanical conditions that correlate with intermittent behavior (temperature, moisture, vibration).

Common Repairs

• Replace the ECM/PCM power relay (and, if warranted, the relay socket or harness connector).
• Repair or replace damaged wiring or corroded connectors in the ECM/PCM power path.
• Replace blown fuses or fusible links and address root-cause (e.g., wiring short, accessory load).
• Replace or repair ignition switch or related control circuits if they fail to energize the relay properly.
• Restore proper battery capacity or replace aging battery/alternator components if voltage supply is insufficient.
• In rare cases, reprogramming or replacing the ECM/PCM if internal faults are confirmed after exhaustive external checks.

Notes on the sources

• The OBD-II overview pages provide the framework for understanding DTCs and how powertrain faults are categorized, which supports the general approach to diagnosing P0689 as a power-relay/ECM-power-supply issue.
• Emissions testing discussions give context for why proper ECM/PCM operation matters for emissions and drivability, reinforcing the importance of resolving power-supply faults.
• If you need more precise, vehicle-specific wiring diagrams and P0689 definitions for a particular make/model, consult OEM service information or a dedicated diagnostic database; the general approach above remains valid across most platforms.

When to Escalate

• If you confirm a healthy relay, correct control signal, and proper power at the ECM/PCM input, but the PCM still does not function or keeps reporting P0689, consider internal PCM faults, software/firmware issues, or an ECU-related failure. This step often requires OEM-level diagnostic tools, software updates, or PCM replacement per manufacturer recommendations.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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