Comprehensive diagnostic guide for OBD-II code P0680

Purpose of this guide

• Provide a safe, step-by-step approach to diagnosing P0680, which is commonly associated with the PCM (Powertrain Control Module) power relay control circuit.
• Use information to ground the approach in standard OBD-II concepts (especially powertrain codes) and to frame expectations about what this code means in the context of modern vehicles.
• Offer practical probabilities for likely causes based on typical field experience when data from NHTSA or other complaint databases isn't included .

What This Code Means

• OBD-II codes are used by modern vehicles to monitor engine and transmission control parameters. They are part of a standardized diagnostic framework that allows the PCM to flag issues in the powertrain (engine/transmission) systems.
• P0680 is widely categorized as a powertrain code related to the PCM power relay control circuit in many open-source/standard code definitions. The exact wording and pin/relay specifics can vary by manufacturer, but the overarching concept is that the PCM's power relay control circuit is not functioning as expected.

Symptoms

• No crank or no start, or intermittent starting behavior
• Engine may crank but fail to start if the PCM never powers or correctly powers itself
• Accessories may experience abnormal power behavior or abrupt loss of power
• DTC P0680 may appear alongside other power/ground-related codes or PCM communication codes
  Note: Symptoms can vary by vehicle and by whether the problem is a relay, wiring, ground, battery condition, or PCM fault.

Safety and initial precautions

• Work with the ignition OFF and the battery disconnected before disconnecting any PCM connectors or wiring harnesses when doing visual inspections or wiring tests.
• If you must energize circuits for testing, keep hands clear of moving parts and hot surfaces, and use insulated tools.
• Avoid backfeeding into the PCM or other controllers; do not bypass relays or fuses without a proven diagnostic reason.
• Observe all standard shop safety procedures, including PPE and lockout/tagout procedures when working on vehicles.

Important tools and resources

• OBD-II scan tool capable of reading DTCs and freeze-frame data; ability to clear codes and monitor data stream for PCM relay status or related system voltage
• Multimeter (digital preferred) for voltage, resistance, and continuity testing
• Test light or infrared thermometer as needed
• Vehicle service manual or wiring diagrams for the exact PCM power circuit, relay location, and connector pinout
• Basic hand tools for fuse/relay access and connector inspection

Step-by-Step Diagnosis

1) Confirm and frame the problem

• Retrieve P0680 with a scan tool and note any freeze-frame data and any other DTCs present.
• Check if the issue is intermittent or continuous (e.g., does the engine crank but not start, or does it fail to power up at all?).
• Confirm vehicle-specific behavior: some vehicles may show "no power to the ECU" or "ECU not getting fuel/ignition power" type symptoms when the PCM relay is not being energized.

2) Inspect power supply basics

• Verify battery condition: battery voltage should be healthy (typically ~12.6V with engine off and adequate cranking voltage when starting). A marginal battery can cause false negatives or intermittent relay operation.
• Check charging system as a longer-term factor; a weak alternator can cause voltage drops that affect the PCM power circuit.

3) Check fuses and the PCM power relay

• Locate the PCM power relay and its associated fuse(s). Inspect the fuse(s) for continuity and replace if blown.
• Check the relay: listen for an audible relay click when the ignition is key-on or when the scan tool triggers the PCM control (if vehicle provides access to relay control status). Use a test light or multimeter to verify coil resistance and that the coil is energizing when commanded.
• If the relay does not energize or if the coil resistance is out of spec, replace the relay and re-test.

4) Inspect the relay control circuit from the PCM

• Test the PCM control pin(s) that drive the power relay coil. With ignition on or when commanded by the scan tool, verify whether the PCM is outputting the correct control signal to energize the relay.
• If the PCM control signal is absent or erratic while the relay coil is known good and other power supplies are present, the issue may be in the PCM relay-control circuit (inside the PCM, a wiring fault, or a bad ground reference).

5) Verify the PCM main power supply and ground

• Main PCM power: verify that the PCM power pin(s) are receiving 12V (or the expected rail voltage) when the ignition is on. This is the supply that the relay would pass to the PCM's internal circuits.
• PCM ground: verify a solid ground at the PCM casing/ground pins. A high-impedance or intermittent ground can cause PCM misbehavior and false relay control faults.
• Check for corrosion, bent pins, or loose connectors at the PCM power/ground pins and at the relay connector.

6) Inspect wiring between the battery/fuse box/relay and the PCM

• Look for damaged insulation, chafed wires, or harness pin damage between the fuse box, the PCM, and the relay. Try to trace the power feed from the battery through the fuse to the relay coil and the PCM supply pin.
• Check for shorts to ground or to other circuits (which can cause relay coil issues or PCM supply faults).
• Confirm that there are no aftermarket modifications that could have altered the power feed or ground references.

7) Check for a PCM-related fault

• If all power and wiring checks out, consider the possibility of an internal PCM fault (power rail regulation, relay drive transistor, or software/firmware issue).
• In some cases, updating or re-flashing PCM software (as directed by the vehicle manufacturer) or replacing the PCM may be required if the control circuitry is defective and no other causes are found.
• Note that PCM replacement/repair can be vehicle-specific and may require dealer tools or service procedures.

8) Confirm the fix and perform post-repair testing

• After any repair (relay, fuse, wiring, or PCM service), clear the DTCs with the scan tool and perform a test drive or key-on/key-off cycles to ensure the code does not reappear.
• Monitor live data if possible: check the relay coil control signal, PCM supply voltage, and ground status during a start attempt to verify the circuit operates as intended.
• Verify that the engine starts reliably and that no new codes are generated.

Common Causes

• Most frequent: Faulty PCM power relay, fuse, or supply wiring (including poor connections at the relay or PCM power pin)
• Next: Damaged wiring or a compromised harness between the battery/fuse block/relay and the PCM
• Next: PCM control circuit fault (PCM output to the relay coil is not functioning even though inputs are normal) or an internal PCM fault
• Less common: Battery condition or charging issues causing insufficient voltage to energize the relay reliably
  Note: These probability estimates reflect typical field experience rather than published NHTSA complaint data, as such data isn't provided in .

Documentation

• Exact vehicle year, make, model, and engine family
• DTCs present (P0680 and any other codes)
• Freeze-frame data (engine load, RPM, voltage, etc.)
• Battery and charging test results
• Fuse and relay part numbers, locations, and test results
• Wiring test results (pin numbers, resistance/continuity readings)
• PCM connector condition (pin integrity, corrosion, alignment)
• Any vehicle-specific notes (factory service manual references, wiring diagrams)

Important Notes

• P0680 definitions and interpretations can vary slightly by manufacturer; the general concept is PCM power relay control circuit related. Always confirm with the factory service information for pinouts and wiring diagrams for the specific vehicle.
• A PCM may be a single point of failure or a distributor of power to other modules; in some vehicles, the power relay controls power to several modules, so a P0680 could appear alongside other power-related issues.
• When replacement is indicated, consider the possibility that a failed relay, wiring fault, or connector problem may cause repeated failures of a new PCM if not addressed.

Resources for further reference

• OBD-II codes are used to monitor various parameters and that powertrain codes are a subset of DTCs used for engine/transmission control. This provides the conceptual basis for diagnosing P0680 as a powertrain power control issue.
• Open-source/open-standards code definitions (as used in many automotive diagnostic communities) align P0680 with the PCM power relay control circuit, though exact wording and pin references can vary by manufacturer. When in doubt, verify with the vehicle's service manual or OEM diagnostic resources.
• If there is conflict between sources, use the vehicle-specific wiring diagrams and service procedures as the final authority.

Troubleshooting quick-reference checklist

• DTC P0680 is present; confirm no conflicting codes that could describe a different power or ground issue
• Battery is healthy and charging system is functional
• Fuse for PCM supply is intact
• PCM power relay coil and contact function: coil energizes when commanded; relay provides 12V to PCM
• PCM control pin to relay coil is driven when ignition is on or when commanded by scan tool
• Main PCM power and ground pins are at proper voltage/ground; no loose connections or corrosion at PCM connectors
• Wiring between fuse/relay and PCM is intact with no shorts to ground or other circuits
• No PCM internal faults; if suspected, verify with proper OEM procedures (software/firmware status, possible replacement)

In summary

• P0680 is a powertrain code tied to the PCM power relay control circuit. The most common failures involve the relay, its fuse/supply, or the wiring/connectors to the PCM. A systematic electrical test of power, ground, relay control signals, and wiring should quickly identify the fault. If all external wiring and relays test OK, consider PCM control circuit faults or PCM itself as possible causes, recognizing that these cases may require OEM-level diagnostics or replacement.

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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