P1478: Secondary Fan Circuit High Voltage
The P1478 code indicates that the Powertrain Control Module (PCM) has detected a high voltage signal on the secondary radiator fan control circuit when the fan is commanded off. In Ford vehicles, this code specifically relates to the secondary electric fan control system, where the PCM monitors circuit status via voltage feedback. The fault condition is triggered when the control circuit voltage remains high while the PCM has not activated the fan.
01. Most Common Causes
- Faulty secondary fan relay with stuck closed contacts
- Failed fan control transistor or driver inside the PCM
- Short to voltage in the secondary fan control circuit wiring
- Secondary fan motor with altered internal resistance causing abnormal feedback
- Corroded or damaged connections in the fan harness leading to erroneous readings
- Failed fan control module (if applicable for the specific model)
02. Symptoms You Will Notice
- Engine overheating, especially in stop-and-go traffic
- Check Engine Light illuminated
03. Step-by-Step Diagnosis
Scan for codes and verify P1478 is present
Inspect the secondary fan relay and wiring for shorts or corrosion
Test the fan motor resistance and operation with a direct power source
Check the PCM fan control output signal with a multimeter or oscilloscope
If all else is normal, suspect a failed PCM driver
1. Frequently Asked Questions
Is it safe to drive?▼
Driving with this code is not recommended as it can lead to engine overheating, especially in traffic or hot weather. Have it diagnosed promptly.
How much does it cost to fix?▼
Repair costs typically range from $150 to $600, depending on whether the issue is a simple relay replacement or a more expensive PCM repair.