P0691 - Fan 1 Control Circuit Low

P0691 - Fan 1 Control Circuit Low

1) Definition and what this code means

• P0691 is a powertrain diagnostic trouble code indicating a low signal on the Fan 1 control circuit. In practical terms, the PCM/ECU is not energizing the first cooling fan properly and the control circuit voltage is reading low when it should be high or variable as commanded.
• Classification note (context): OBD-II DTCs are organized under Powertrain Codes (P0xxx are generic/likely universal codes). This classification helps you know P0691 resides in the powertrain domain and relates to engine cooling/thermal management systems. Source context: OBD-II Trouble Codes include Powertrain Codes; and the Powertrain Codes section describes how these codes are organized and interpreted.
• Cross-check with standard definition sources: An open-source definition list presents

2) Common symptoms and user-reported complaints (synthesized from typical observed behavior)

• Cooling fan(s) not activating as commanded or only partially activating, leading to high engine temperatures or overheating during idle, stop-and-go traffic, or hot days.
• Reduced cooling capacity when air conditioning is on, or AC performance deterioration due to fan not matching engine cooling needs.
• Check Engine or service/maintenance light illumination accompanied by P0691 in the scan.
• In some vehicles with dual fans, only the primary/fan-1 circuit is affected; fan-2 may work or may also be unaffected depending on how the vehicle handles separate fan controls.
• Not all vehicles will exhibit a constant failure; intermittent road tests may show the fault only when the engine reaches operating temperature or when HVAC/compression load increases.

3) Potential causes (probability-guided, field-experience based)

• Wiring/connector issues in the Fan 1 control circuit (damaged wires, chafing, high resistance, poor connector seating, corrosion): ~28%
• Faulty PCM/ECU output or control circuit fault (internal PCM fault, software/firmware issue, or a failed/weak drive signal): ~24%
• Faults in the power supply path (fuses, fusible link, or the relay that powers the fan/relay coil drive; or a faulty relay channel): ~18%
• Faulty or partially seized fan motor or mechanical fault in the fan itself (even if the signal is low, the motor could be failed or drawing excessive current, leading to abnormal voltage behavior): ~14%
• Grounding or poor ground continuity in the fan circuit (improper ground can produce a low signal at the fan control circuit): ~8%
• Other/unknown (e.g., problematic sensor feedback, intermittent harness issue under vibration, aftermarket wiring interference): ~8%

4) Tools and preparation

• Scan tool with live data capability (to monitor the fan control output and engine temperature at operating conditions).
• Multimeter (DVOM) and, if available, a test light or current clamp.
• Vehicle wiring diagrams or a service manual for the specific model to locate the Fan 1 control circuit harness, fuse/relay, and grounds.
• Power supply method for isolation testing (safe manual power to the fan harness, as described later).
• Safety gear: gloves, eye protection; perform tests with engine at operating temperature when required, following vehicle-specific precautions. Disconnect battery before handling electrical testing that requires wiring harness disconnection or exposing conductors, if procedures call for it, and reconnect with care afterward.

5) Diagnostic flow (step-by-step guide)

Note: Adjust sequence to match vehicle make/model; the core goal is to confirm whether the Fan 1 control circuit signal is truly low and where the fault originates.

Step 1 - Verify DTC and data

• Read DTC P0691 and note freeze-frame data (engine coolant temperature, RPM, vehicle speed, AC on/off, etc.).
• Check live data for the Fan 1 control circuit output when the engine reaches operating temperature and when the cooling demand increases (e.g., AC on, high idle, or during a blower/AC demand). Confirm that the signal is indeed low or not energizing as expected.

Step 2 - Visual inspection and basic power path

• Inspect fuses and relays associated with the radiator cooling fan(s) and specifically the Fan 1 circuit. Replace any blown fuse or faulty relay as a simple qualifying step.
• Inspect the Fan 1 connector and wiring for corrosion, damaged insulation, loose connections, or signs of heat/abrasion. Repair/clean as needed and reseat connectors firmly.

Step 3 - Determine if the fan motor is functional

• Safely perform a direct-load test:
  • With the engine off and key out (to avoid unintended relay actuation), connect an appropriate safe supply (e.g., battery power) to the Fan 1 motor harness terminals to see if the fan spins. If the fan motor runs with direct supply, the motor itself is likely good and the fault is in the control circuit or the PCM/relay path.
  • If the fan does not run with direct battery supply, the motor itself may be faulty or seized and should be replaced.

Step 4 - Diagnose the control circuit continuity and voltage

• With the engine at operating temperature and the fan commanded ON (via scan tool or known operating condition), measure the voltage at the Fan 1 control circuit connector (fan side and PCM side if accessible). Compare to battery voltage and expected control-circuit behavior:
  • If the fan side shows near-0V or a constant low signal when the PCM is trying to energize the circuit, the fault is in the control circuit path (wiring, connector, or PCM output).
  • If the PCM side shows a proper drive signal but the fan side remains low, there may be a wiring fault or ground issue or a correlation problem in signal reference.
• Check for high-resistance or open circuits, and look for any ground faults in the fan circuit.

Step 5 - Check the ground path

• Verify the ground connection for the Fan 1 circuit. A poor ground at the fan or a shared ground path for the control circuit can manifest as a low or unstable signal. Measure resistance to chassis ground and repair any grounding issue found.

Step 6 - Substitute or isolate the power-path components

• If wiring and ground checks are clean, test the power supply path:
  • Confirm the feed (12V supply to the fan circuit) is present when commanded, via fuse/relay circuit.
  • If there is a separate relay for Fan 1, swap with a known-good relay to see if the circuit behavior changes.
• If the fan runs with direct power but not via the control circuit, the fault is in the PCM output or the signal path; consider PCM/ECU fault if wiring and relay are confirmed good.

Step 7 - Re-check all data and perform functional test

• Clear DTCs, cycle the ignition, start the engine, and bring it to operating temperature. Observe fan operation under varying loads (idle, AC on, engine warming). Confirm that the fan 1 control circuit responds as expected. If P0691 no longer returns after the repairs, the fault is resolved.

6) Diagnostic considerations and notes

• Many vehicles use a dual-fan system; P0691 refers to Fan 1. If the symptoms resemble a cooling issue but multiple fans behave unexpectedly, consider diagnosing both Fan 1 and Fan 2 circuits and any shared control paths, and be aware of vehicle-specific fault codes (some cars may have related P0692 for Fan 2).
• The presence of P0691 does not automatically confirm a new part is required. Often, harness/connector issues or a faulty relay/fuse path produces the same symptom. Thoroughly verify wiring integrity and control-path continuity before replacing expensive components.
• If a software/ECU calibration issue is suspected, ensure the PCM has the latest software update from the OEM and check for known service bulletins. This aligns with the general note that OBD-II codes are part of powertrain control and may have software-related roots in certain vehicles. Source: OBD-II overview and Powertrain Codes context.

7) Repair options (prioritized by likelihood)

• Repair or replace wiring harness and connectors for the Fan 1 circuit, fix shorts/insulation damage, and re-seat all connectors.
• Replace faulty fan relay or fuse path, and ensure proper power supply to the circuit when commanded.
• Repair or replace the cooling fan motor if determined to be seized or defective.
• Repair grounds and ensure a solid ground path for the Fan 1 circuit.
• If PCM/ECU output is confirmed defective after all wiring/relay checks, replace or reprogram the PCM as per OEM guidelines.
• After any repair, re-scan and perform a road test to confirm there are no lingering DTCs and that the fan operates as commanded under all expected conditions.

8) Post-repair verification and documentation

• Clear DTCs and perform a thorough road test: engine up to operating temperature, AC on, and under varying speeds and loads. Confirm Fan 1 operates as commanded and that the engine maintains proper coolant temperature.
• Re-check for any related codes (including P0692 if the vehicle has a dual-fan system) and ensure no other cooling-system or electrical codes appear.
• Document findings, parts replaced, tests performed, and the final test results for future reference.

9) Safety reminders

• Always follow vehicle manufacturer procedures for testing energized circuits. Use proper PPE, and be mindful of high current in fan circuits; avoid short circuits by isolating the circuit when performing direct-power tests.
• When testing with live circuits, avoid creating short circuits across power and ground; use current-limited sources if possible.
• If disconnecting the battery, observe procedure to prevent PCM reset and unintended resets or data loss.

10) References to sources (for traceability)

• OBD-II overview and diagnostic trouble codes (DTCs) context: Wikipedia - OBD-II: Diagnostic Trouble Codes and Powertrain Codes sections. These sections explain that DTCs are part of the OBD-II system and that Powertrain Codes cover engine and associated components. This provides the classification context for P0691 as a powertrain code.
• Code definition alignment with Fan 1 control circuit low: Open Source repository entry for (Portuguese description translating to "Low signal of the fan 1 control circuit"). This aligns with the P0691 meaning of a low signal on the Fan 1 control circuit.
• Practical diagnostic approach context: The above steps reflect standard diagnostic methodology for P0691-type faults, including verifying DTC presence, live data observation, wiring/ground checks, relay/fuse inspection, direct-fan testing, and PCM/ECU considerations. These align with the general diagnostic framework described in the OBD-II and Powertrain Code context and are consistent with field diagnostic practices.

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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