Comprehensive diagnostic guide for OBD-II code P0694

What This Code Means

• Meaning (based on Open Source code definitions): P0694 is described as a high signal on a fan control circuit (the exact fan number can vary by manufacturer). In practical terms, the ECM/PCM is seeing the control circuit for a cooling fan as being commanded ON (high) when it should not be, or it is reporting a fault on the circuit that drives the fan. The provided Open Source listing translates to .
  • Note: Exact fan-number designation (e.g., Fan 1, Fan 2) and the intended normal state can vary by vehicle make/model. When diagnosing, identify which fan/relay the vehicle uses for coolant cooling and confirm the expected behavior from the factory service information.
• Where it sits in the system: P0694 is a Powertrain/OBD-II diagnostic trouble code. This category covers emissions-related and engine/fuel/systems controls, including cooling fan control logic (per the OBD-II framework). Source context: OBD-II codes are Powertrain codes that the ECU monitors and reports when a fault is detected.

Symptoms

• The cooling fan stays on or comes on unexpectedly (even when the engine is cool or the ignition is off, depending on vehicle wiring and control wiring).
• The check engine light (or MIL) is illuminated with P0694 stored or pending.
• Possible battery drain or faster-than-normal fuel consumption when the fan remains energized or the circuit is erratic.
• In some cases, the engine may experience temperature regulation issues if the control circuit malfunctions in a way that affects proper fan engagement timing (though the primary diagnostic flag is the high signal in the fan control circuit).

Common-cause probability (practical, field-oriented estimates)
Note: Specific NHTSA complaint frequency data for P0694 is not provided in . The following probabilities are grounded in typical OBD-II fan-control symptoms and common wiring/driver issues observed in the field, and are intended as a practical guide rather than a definitive statistical breakdown.

• Short to power on the fan control circuit or a welded/shorted relay (fan relay stuck closed): 25-40%
• Faulty or failing fan control relay or driver transistor inside the PCM (PWM/driver circuit stuck high): 15-25%
• Wiring harness damage, loose/corroded connectors, or ground issues on the fan control circuit: 10-20%
• Faulty PCM output, software/calibration issue, or a damaged control module: 5-15%
• Faulty cooling fan motor or a high-resistance/wiring issue to the fan itself: 5-10%
• Other intermittents (short to another circuit, sensor misreadings affecting the control logic): 5-10%

Tools Needed

• OBD-II scan tool (with live data and freeze-frame capability)
• Multimeter (DVOM) and power probe for voltage/current checks
• Basic hand tools (screwdrivers, socket set) for harness and relay checks
• Infrared thermometer or inlet/outlet temp checks to corroborate cooling behavior (optional)
• Safe work practices: disconnect battery before bench-testing electrical components if you plan to remove wiring harnesses or relays; never probe live high-current circuits with exposed metal tools.

Step-by-Step Diagnosis

1) Confirm code and corroborate with freeze-frame data

• Verify that the DTC present is P0694 and review freeze-frame data for engine temperature, vehicle speed, A/C status, cooling fan commanded state, and RPM at the moment the fault was captured.
• Check for any additional codes that might point to a common cause (e.g., improper grounds, relay failures, or other fan-related codes). Source context: DTCs are a structured part of OBD-II powertrain monitoring.

2) Symptom confirmation and live data

• With the engine and vehicle in appropriate running conditions, monitor live data:
  • Fan 1 and Fan 2 (or the relevant fan(s)) commanded state
  • Coolant temperature
  • Engine load, RPM
  • Battery voltage
  • A/C status (if applicable)
• Confirm if the PCM is commanding the fan circuit high (ON) when it shouldn't be, or if the signal is erratic/inconsistent. If the fan is commanded ON but not energizing, that points to a different set of symptoms (drive signal vs. power path).

3) Visual and basic electrical check

• Inspect the fan harness connectors for signs of corrosion, bent pins, or moisture. Check for chafed wiring against sharp edges.
• Check fuses and relays related to the cooling fan circuit. Replace a suspect relay with a known-good unit to rule out a flaky relay.
• Inspect grounding points for the fan circuit and the PCM ground; corrosion or a high-resistance ground can produce abnormal control signals.

4) Determine whether the fault is in the control signal or the power path

• Signal-path test (control circuit diagnostic):
  • With the engine off, examine the control circuit wiring for voltage on the fan control pin when the ignition is on but the engine is not running. A solid high signal here (without a corresponding fan operation) supports a control-path fault or a short-to-power condition.
  • If the PCM is expected to drive a PWM signal, verify the signal integrity (voltage, duty cycle) with an oscilloscope or suitable data tool if available.
• Power-path test (fan/relay/power supply):
  • Bypass the control circuit temporarily by wiring 12V directly to the fan's power input, with a proper fused path and ground. If the fan runs normally, the issue likely lies in the control circuit or PCM drive signal rather than the fan motor itself.

5) Inspect the fan relay and driver path

• Test the fan relay coil resistance and operation (use a low-current test lamp or ohmmeter). Swap with a known-good relay if available.
• With a turn-signal test or service-mode test (per the vehicle's service data), determine if the PCM correctly switches the relay. If not, the issue may be PCM output or software.

6) Inspect the fan motor and load

• If the fan does not run when directly powered (step 4), inspect the fan motor windings for continuity and ground. A seized or high-resistance motor will not draw proper current even if powered, indicating a failed fan motor.

7) PCM/ECU considerations

• If all wiring, relays, and the fan motor test OK, but the PCM output still shows a high signal or incorrect state, consider PCM software/firmware updates per the vehicle manufacturer, or PCM replacement if diagnosed as faulty output driver. Note that PCM replacement is a more involved repair and typically requires programming.

8) Document and cross-check

• Record voltages, resistance readings, and the observed behavior under test. Cross-check findings with manufacturer service information (vehicle-specific wiring diagrams and service bulletins). The exact fan-number designation and commanded behavior can vary by make/model.

9) Repair actions (typical, in order of likelihood)

• Replace the faulty relay or fix a corroded/loose harness connector in the fan control path.
• Repair or replace damaged wiring in the fan control circuit (chafed insulation, corrosion, broken conductor, poor ground).
• Repair/replace the PCM output driver or reprogram/flash the PCM if there is a fault in the control circuitry or software.
• Replace the cooling fan motor if it fails to operate when power is applied directly (i.e., motor fault).
• After any repair, clear codes and perform a road test or drive cycle to confirm that the fault does not return.

Post-Repair Verification

• Clear the P0694 code and drive the vehicle under normal operating conditions (coolant at normal temperature, various engine loads) to verify that the fault does not reappear.
• Confirm that the fan behavior matches the expected design (i.e., fan engages at appropriate coolant temperature or commanded conditions and remains off when not needed).
• Re-check for any new DTCs that might have been triggered by the fault or repair.

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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