Comprehensive diagnostic guide for OBD-II code P0681

Note: The available sources do not provide a canonical OEM definition for P0681. In OBD-II practice, P0xxx codes are powertrain codes (PCM/engine control related) as noted by Wikipedia. The Keep Alive Power (KAPWR) or similar keep-alive/PCM power-supply terminology is commonly associated with P0681 in field experience, but OEM wording can vary. The guide below uses the general interpretation that P0681 points to the PCM's keep-alive power circuit having a fault (low voltage, loss of supply, or poor connection). If your vehicle uses different terminology, cross-check with the factory service information for your make/model.

What This Code Means

• Based on the generic OBD-II framework, P-series diagnostic trouble codes are powertrain codes (P0xxx are generic across many makes) per the OBD-II standard. This places P0681 in the PCM/Powertrain category and typically links it to the Powertrain Control Module's keep-alive or essential power supply circuit. Wikipedia describes P0xxx codes as powertrain codes and emphasizes that the monitoring system reports faults when parameters exceed thresholds. This informs that P0681 implicates the PCM's power supply circuit rather than a single sensor or actuator fault.

Important Notes

• The above interpretation aligns with the general structure of OBD-II codes as described by Wikipedia. The provided entry is not directly applicable to P0681 (it relates to glow plugs in a separate context). If a vehicle-specific definition for P0681 is published by the OEM, follow that definition first.

Symptoms

• Engine may crank but fail to start or run, especially after a power loss or ignition cycle.
• No PCM communication on scan tool or intermittent loss of communication with the PCM (possible U-codes alongside P0681).
• MIL on or flashing in cases where voltage dips affect other control modules.
• Cranking may be slow or inconsistent; the ignition switch may seem to momentarily lose power to the PCM.
• Battery light or charging system indications if the KAPWR supply is tied to vehicle power (especially when the alternator is stressed).
• Intermittent failures that recover after a complete ignition cycle or after restoring power (e.g., disconnecting/reconnecting battery).

Probable Causes

Note: No NHTSA or OEM defect-complaint dataset is provided . The probability estimates below reflect ASE-field experience and common fault patterns for KAPWR-type issues, expressed as approximate ranges:

• Faulty KAPWR path (relay, fuse, or connector to the PCM) - 40-50%
  • A sticking or welded-in KAPWR relay, blown/weak fuse, or corroded/loose connectors in the PCM power feed are the most frequent causes.
• Battery, alternator, or grounding issues causing unstable supply - 25-35%
  • Low or fluctuating voltage can manifest as intermittent loss of PCM power even when the engine starts; parasitic draws can also compromise KAPWR.
• Wiring harness damage or poor PCM power connector (pin-to-pin resistance, shorts, or grounding problems) - 15-25%
  • Damaged insulation, chafing, or pin corrosion can drop voltage to the PCM or create intermittent ground paths.
• Ignition switch or power feed path to the PCM (OEM supply path or ignition relay) - 5-15%
  • A worn ignition switch or degraded feed to the KAPWR circuit can mimic a faulty KAPWR relay.
• Aftermarket devices, immobilizers, or security systems disturbing KAPWR - 5-10%
  • Some adds-on or factory security integrations can intermittently cut power to PCM or alter timing of the KAPWR feed.
• Less common: PCM internal fault (power supply regulation inside PCM) - 0-5%
  • PCM regulator failures are rare but possible; usually accompanied by other PCM fault codes.

Diagnostic Approach

Prepare safely

• Ensure personal safety: wear eye protection and gloves when working near batteries and high-current circuits. Disconnect power only as required for certain tests, following OEM guidance to avoid data loss or damage.
• Gather tools: DVOM/OLM tester, test leads, known-good 12V battery, vehicle service information (wiring diagrams for the PCM power circuit), scan tool, and, if available, a load tester for the battery.

Step 1. Confirm and document the fault

• Use a capable scan tool to pull all DTCs, freeze-frame data, and any pending codes. Note any U-codes (communication) that appear with P0681, as those can indicate broader power or network issues.
• Record observed symptoms (cranks, no start, intermittent behavior, vehicle age, battery/alternator status).

Step 2. Baseline power and ground checks

• Measure battery voltage with engine off: target 12.6 V or higher.
• Measure battery voltage with engine cranking: voltage should remain above ~9-10 V for the ECM to function; excessive drop suggests battery or heavy load issues.
• Check chassis and PCM grounds for corrosion, looseness, or high resistance. A bad ground can mimic KAPWR failure.
• If the vehicle has a parasitic draw or a known charging issue, address that first to isolate KAPWR symptoms.

Step 3. Identify the PCM power feed components to test

• Locate the KAPWR (keep-alive power) circuit: this is the PCM's primary power feed that remains vital for memory and engine control logic. Identify related fuses, fusible links, and relays in the power feed path to the PCM (reference OEM wiring diagrams).
• Common items to inspect:
  • KAPWR fuse or power fuse for the PCM
  • KAPWR relay and its coil supply
  • Wiring harness between battery/ignition and the PCM power input
  • Fusible links and any inline fuses in the PCM power path
  • Ignition switch feed to the KAPWR circuit

Step 4. Measure the KAPWR circuit live

• With the ignition in RUN (or as specified by the OEM for your vehicle), measure voltage at the PCM power input pin(s):
  • Expect near battery voltage (12.6V+) at rest; during RUN it should be stable and typically in the 12.5-14.8V range depending on alternator load.
  • If voltage at the PCM power pin drops significantly (below ~11V under load) or oscillates with engine cycling, suspect a voltage drop in the KAPWR path (relay, fuse, or wiring).
• Check the KAPWR relay operation:
  • Relay coil should energize with key ON.
  • Contact side should show near battery voltage feeding the PCM power pin when energized.
  • If the relay buzzes, sticks, or fails to energize, replace or service the relay and inspect its control circuit.

Step 5. Inspect wiring and connectors

• Inspect PCM power input pins and their connectors for corrosion, bent pins, or water intrusion.
• Check for damaged insulation or pin looseness; reseat and reseal as necessary.
• Wiggle test the harness while monitoring PCM power rail to identify intermittent contact issues.

Step 6. Evaluate the battery and charging system

• Perform a load test on the battery to ensure it can sustain the necessary current during cranking and PCM operation.
• If the alternator is weak, voltage may sag during engine load and affect KAPWR; ensure alternator output is within spec (i.e., ~13.5-14.8V when running).
• Check for parasitic drains that may pull the KAPWR circuit down when the vehicle is off.

Step 7. Review aftermarket and immobilizer/ security interactions

• If an aftermarket alarm, remote starter, or major wiring modification exists, verify that it is not cutting power to the PCM's KAPWR circuit or interfering with ignition timing.

Step 8. Clear codes and re-test

• After addressing any faults found (fuse/relay/wiring/ground/battery), clear codes and perform a road test or startup test to confirm P0681 does not reappear.
• Re-check for any new codes that may appear after the repair; P0600-P0604 (communication errors) or P067x (glow plug-related) can co-occur in some vehicles but are separate issues.

Step 9. If P0681 persists

• Consider OEM service information: some vehicles may have a separate keeper relay or an alternate feed path to the PCM; a voltage drop test on the entire KAPWR path may reveal a sub-system fault not captured by a single-point voltage test.
• If the KAPWR circuit test shows consistent voltage delivery but P0681 persists and no wiring faults are found, PCM internal fault or firmware/coding issues could be a contributing factor. In such cases:
  • Confirm calibration/version of PCM software with OEM service bulletins (SBs) for KAPWR behavior.
  • In rare cases, PCM replacement or reprogramming may be required, following OEM procedures.

Documentation

• Document all measured voltages, test results, connector condition, fuse/relay part numbers, OEM wiring path references, and any observed intermittent behavior.
• Photograph damaged components or connectors, and keep a log of fault occurrence times and environmental conditions.

Documentation

• P0681 indicates a problem with the PCM's power-supply path (keep-alive power). The most common fixes are usually wiring/relay/fuse issues, battery/charging problems, or a poor PCM power connection.
• Repairs often involve repairing or replacing a fused link, relay, or connector, followed by re-testing and clearing codes.
• If a PCM fault is suspected, OEM-specific software updates or PCM replacement may be necessary.

Safety and compliance notes

• When working around the battery and high-current circuits, ensure the ignition is off before disconnecting cables to avoid arcing.
• If you must test under load, use appropriate PPE and follow vehicle service information to avoid damaging sensitive electronics.

How industry sources frame this area (source context)

• Diagnostic Trouble Codes in OBD-II are generated by the vehicle's monitoring system when a parameter exceeds its allowable range. This supports the premise that P0681 is a PCM/Powertrain-related fault tied to the power supply to the PCM.

• OBD-II powertrain codes categorize faults that involve engine and emissions control systems, reinforcing that P0681 belongs to the PCM or its power supply pathway rather than sensor-specific failures.

• If a user reports intermittent no-start and power loss that recovers after cycling ignition, a KAPWR issue (relay, fuse, or wiring) is highly probable (25-50% range depending on vehicle history).

• Battery or charging problems are a close second cause, given the dependence of PCM on stable voltage (roughly 25-35% likelihood).

• Electrical harness damage or a degraded PCM power connector is plausible (15-25%), especially on older vehicles or vehicles with harsh conditions (vibration, moisture).

• Ignition switch or security/immobilizer interference is less common but possible (5-15%).

References (where applicable)

• Note: The Glow Plug/ Cylinder 11 entry provided in the Open Source repository is not directly applicable to P0681. It demonstrates the diversity of DTC naming in some repositories but should not be assumed to define P0681.

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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