OBD-II Diagnostic Trouble Code
P0691 — Fan 1 Control Circuit Low
P0691 means the control circuit for cooling fan 1 is reading low voltage. On trucks with an electronically controlled fan (air-operated fan clutch solenoid, electric fan relay or variable-speed fan drive), the ECM saw the driver output low, typically from a short to ground in the control wire or solenoid.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Fan 1 Control Circuit Low
Code
P0691
Category
Powertrain
Subsystem
Cooling
Truck-relevant
Yes
What this means for truckers
P0691 (Fan 1 Control Circuit Low) — fan control output shorted to ground.
Before you replace parts — try this first
P0691 on the Cooling system is most often fan clutch solenoid or fan relay control wire shorted to ground — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Depending on fan drive design the fan may be stuck engaged (loud fan roar, reduced fuel economy) or not engage (rising coolant temperature, overheating on grades or at idle with A/C on). Many heavy-truck air fan clutches are spring-engaged and fail with the fan on. The check-engine light is on and some engines derate on high coolant temperature.
Common causes
- CommonFan clutch solenoid or fan relay control wire shorted to ground ($120–$500)
- CommonFailed fan clutch solenoid valve or electric fan relay coil (internally shorted) ($150–$700)
- OccasionalCorroded or damaged fan connector near the front of the engine ($80–$300)
- RareECM fan driver fault ($1200–$3500)
How to fix
Inspect the fan connector and harness along the radiator and engine front for chafing
When to use: First, since this harness sees heat and vibration
Measure solenoid or relay coil resistance and check for a short to ground
When to use: When harness appears good
Use a scan tool to command the fan on/off while monitoring the circuit and fan response
When to use: To confirm the repair and isolate the component
Check the air supply to a pneumatic fan clutch and the clutch itself
When to use: When the electrical circuit tests good but fan operation is wrong
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0691?
P0691 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0691 on a commercial truck?
Total repair cost ranges $80–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Fan clutch solenoid or fan relay control wire shorted to ground — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0691 fail a DOT inspection?
P0691 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0691 and related codes?
P0691 is closely related to: P0670 (Glow Plug Control Module Circuit), P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit). These codes share the same subsystem (cooling) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0691 is set without the companions, the issue is more isolated.
How do I prevent P0691 from coming back after a repair?
For P0691 (Fan 1 Control Circuit Low), prevention centers on the cooling system. Service cooling system at OEM interval. Use the correct coolant type — modern HD engines need ELC (Extended Life Coolant), not green/conventional. Pressure-test cap and system annually.