Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Chart C12A - Coolant Fan Always On
Inputs from the coolant temperature sensor are used by the ECM to control low and high speed coolant fan operation. Other inputs that are used by ECM are the A/C control switch, A/C pressure switch and VSS. The ECM controls cooling fans through the grounding of circuit No. 535 (low speed fan relay), or circuit No. 536 (high speed fan relay). When the high speed fan relay is activated, low speed fan relay is activated simultaneously.
The Low speed fan relay will activate under the following conditions:
- A Code 14 or 15 is set.
- Coolant temperature 217°F (103°C), or more.
- A/C clutch is engaged with vehicle speed less than 35 MPH.
High speed fan relay engages regardless of vehicle speed if:
- No Code 14 or 15 is set.
- Coolant temperature is 244°F (118°C), or more.
- A/C pressure is high.
The coolant fan will be commanded on high speed when engine is not running, and coolant temperature is 239°F (115°C) or more. Cooling fan will run for about 7 minutes (only immediately after key is turned off).
- This test determines if circuit No. 535 is shorted to ground. If shorted to ground, relay would be energized at all times.
- This test determines if circuit No. 536 is shorted to ground. An illuminated test light indicates that wire is shorted to ground. The following steps will isolate the short.
- If test light is off after disconnecting ECM, an internal short exist in the ECM. Before replacing ECM, check resistance value of low speed side of fan control relay. If resistance is less than 20 ohms, relay should be replaced. Check that circuit No. 535 is not shorted to battery voltage. Replace canister purge solenoid, if resistance is less than 20 ohms.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.