Comprehensive diagnostic guide for OBD-II code P0690

ECM/PCM Power Relay Control Circuit (with notes on conflicting definitions)

Overview

• Primary definition (common in many OEM and generic OBD-II references): P0690 indicates a fault in the ECM/PCM power relay control circuit. This generally covers issues related to the power supply to the ECM/PCM via its power relay, the relay itself, the control signal from the PCM to the relay, or related wiring/ground issues. In other words, the PCM is not reliably receiving or providing power as commanded, which can cause intermittent or permanent loss of PCM power.

• Alternate/Conflicting reference found in an open-source GitHub entry (translated from Portuguese): which translates to This perspective suggests a fault condition related to a sensor/signal monitored by the relay or its control circuit that could be interpreted differently by some OEMs or tool definitions.

  • OBD-II overview and diagnostic trouble codes for context on DTCs and powertrain codes.
  • OBD-II: Powertrain Codes for the broader category and monitoring context.

• Practical note: Because the GitHub entry presents a different interpretation, it's important to consider vehicle-specific definitions. If your vehicle's OEM service information defines P0690 differently, follow the OEM definition and diagnostic procedure for that model.

Symptoms

• MIL (Check Engine Light) illuminated with P0690 stored or pending.
• Vehicle experiences intermittent no-crank or no-start conditions, especially when PCM power is intermittently cut or not energized.
• Engine may start normally at times, then stall or die if PCM power is interrupted.
• Intermittent electrical symptoms such as dim dash lights, accessory electricals flickering, or irregular gauge operation when attempting to start or during driving.
• After battery boost or jump-start, the vehicle may briefly run but reverts to issues if the power relay control circuit isn't stable.
• In some cases, fuel pump operation or other PCM-powered systems may not run reliably if the ECM/PCM is not powered properly.

Probable Causes

Note: The following probabilities are rough guidance for planning diagnostics and repairs. They reflect typical field observations rather than a statistically proven distribution.

• Faulty ECM/PCM power relay (sticking contacts, coil fatigue, or weak relay) - 40-55%
• Faulty or corroded wiring/connectors related to the ECM/PCM power supply or relay control circuit (pins, grounds, harness abrasion) - 25-35%
• Blown fuse, fusible link, or poor main power supply circuit to the ECM (including battery-to-ECM feed) - 10-15%
• Poor battery voltage or charging system issues causing insufficient voltage to energize the relay reliably (or voltage dips during cranking) - 5-15%
• Faulty ECM/PCM internal power circuitry or a PCM fault (rare but possible) - 5-10%
• Vehicle-specific variations (depending on OEM, the definition of P0690 and the wiring may differ); always cross-check OEM service information.

Safety and preparation notes

• Work in a well-ventilated area; disconnect the battery when performing electrical checking or wiring repairs on the ECM power supply circuits to prevent shock, arcing, or ECM damage. Reconnect only with power removed and key off, following OEM procedures.
• When probing power circuits, use properly rated test equipment (multimeter, load-tested test lights, or a scope) and avoid shorting circuits.
• Be mindful of airbag and high-voltage systems on hybrids/electrified vehicles; follow the manufacturer's safety procedures for those platforms.

Tools Needed

• Basic automotive diagnostic scan tool able to read live data and view PCM relay control status (or a scope for relay coil voltage).
• Digital multimeter (DMM) with good current and voltage measurement capability.
• Service information or wiring diagram for the specific vehicle (to identify the ECM power supply circuit, main fuse/fusible links, relay location, and ground points).
• Jumpers or a known-good power relay (for swap testing).
• Insulation tape, dielectric grease, and standard electrical repair supplies.

Diagnostic Approach

1) Confirm the code and symptoms

• Use an OBD-II scanner to confirm P0690 is present, and note any related codes (P0690 often appears with other power or relay-related codes).
• Note vehicle make/model and any OEM-specific P0690 diagnostic notes if available.

2) Inspect basic power system health

• Check battery condition: voltage at rest should typically be around 12.6V or higher. When cranking or under load, voltage should not drop far below 9-10V. If voltage is low, resolve battery/charging issues first.
• Check alternator and charging system function. A poor charging system may cause intermittent power issues that resemble relay failures.

3) Visual and connector inspection

• Inspect ECM power relay and its socket for signs of wear, corrosion, bent/loose pins, or overheated plastic.
• Inspect harnesses and connectors associated with the ECM power feed and relay control circuit. Look for damaged insulation, chafed wires, burned connectors, or water intrusion.
• Check for blown fuses and fusible links in the ECM power circuit; replace with the correct rating if found.

4) Locate and test the ECM power relay circuit

• Identify the ECM power relay (location varies by vehicle). Determine which circuit supplies power to the ECM (often a hot feed) and which terminal the PCM uses to energize the relay coil.
• Test relay operation:
  • With ignition on (engine off), listen for relay click (if audible) and verify coil voltage at the relay control terminal when commanded by the PCM. If the PCM should energize the relay, you should see 12V (or near) at the coil activation side when commanded.
  • If the relay fails to energize, test the control circuit from the PCM to the relay coil (check wiring resistance, continuity, and ground). If the PCM is not providing a signal, the issue may be PCM-related or a signal path issue.
• If you have a spare known-good identical relay, swap to verify whether the relay itself is the failure.

5) Verify power supply to the ECM

• Check the feed to the ECM power input while the ignition is on and during cranking:
  • There should be a stable supply voltage at the ECM power input (as specified in the service information). If voltage is present with ignition on but drops under load or during cranking, there may be a short or resistance in the power feed or a defective fusible link.
• If the ECM power input is not receiving voltage when it should, isolate the fault to the fuse/ fusible link, main power relay, or harness. Repair/replace as needed.

6) Check ECM ground and related circuits

• Verify the ECM ground is solid and free of resistance. Poor grounding can cause the PCM to misinterpret relay control signals or fail to energize the relay properly.
• Inspect ground points for corrosion or poor attachment. Ensure grounds are clean and secure.

7) Consider PCM control signal vs. PCm power

• If the PCM control signal to energize the relay is present but the relay does not energize, suspect relay or wiring between PCM and relay coil.
• If the relay energizes but the ECM power input still shows no stable voltage to the ECM, suspect a main power feed or fusible link issue beyond the relay.

8) Special notes and cross-checks

• If multiple circuits rely on the same main power relay, failure in that relay can cause multiple systems to act erratic. Confirm whether the same relay feeds multiple ECU modules or systems; some vehicles fuse or combine feeds, which can complicate diagnosis.
• After any electrical repair, clear the PCM fault codes and perform a confirmation drive to verify the condition does not recur.

Testing procedures and expected results (practical checkpoints)

• Relay coil test:
  • Command the PCM (via scan tool) to energize the ECM power relay; you should hear a distinct relay click and measure approximately 12V across the coil terminals (one side must be connected to a reference voltage/ground and the other to the PCM-control side). If no click or no voltage is observed when commanded, the control circuit or PCM may be faulty.
• Power feed test to ECM:
  • With ignition on, measure the voltage at the ECM power input pin. You should see a steady voltage. If this voltage is absent, trace back to fuse, fusible link, main power relay, or wiring faults.
• Ground test:
  • Measure resistance from ECM ground pin to vehicle chassis ground; it should be very low (near 0 ohms). High resistance indicates a grounding issue.
• If available, monitor PCM relay control signal (production data mode) and relay energize status with a scope or live data. Absence of a commanded signal or inconsistent signal indicates control circuit problems.

Repair Options

• Replace ECM power relay if confirmed faulty (no energize or no continuity with coil energized).
• Repair or replace damaged wiring or connectors in the ECM power feed or relay control circuit; cut back insulation, fix corrosion, replace damaged pins, ensure proper contact resistance.
• Replace blown fuse or fusible link in the ECM power supply circuit with the OEM-rated part.
• Repair or replace the ECM if the internal power circuitry is identified as failed (rare; verify no external faults first).
• If a poor battery or charging system is causing voltage dips, repair the battery or charging system, and re-test after replacement.
• Re-test the system after repairs and clear DTCs; perform a roadside or eventual drive cycle to confirm stability.

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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