Radiator fan not coming on — engine overheating in traffic

Good news — this is very diagnosable. The fan circuit has 4 components. Test them in order from cheapest to most expensive: **1. Fuse ($1)** — Check the radiator fan fuse in the under-hood fuse box. The '06 Civic has it clearly labeled on the lid. **2. Relay ($10-15)** — The fan relay is in the same fuse box. Swap it with another identical relay in the box (horn relay is usually the same part number). If the fan starts working, buy a new relay. **3. Coolant temp sensor/switch ($15-25)** — This tells the ECU when to turn on the fan. You can bypass it for testing: unplug the sensor and jump the two wires in the connector. If the fan kicks on, replace the sensor. **4. Fan motor ($80-150)** — If fuse, relay, and sensor are all good, apply 12V directly to the fan motor connector. If it doesn't spin → dead motor. The [2006 Civic wiring diagrams](/wiring/2006/Honda/Civic) have the complete cooling fan circuit showing every connector, fuse, and relay in the path. Makes tracing really easy. **Emergency tip:** Until you fix it, turn your heater to MAX HOT with the blower on high when you're stuck in traffic. The heater core acts as a mini radiator and will keep the temp down.

On the 8th gen Civic specifically, the fan relay solder joints on the fuse box circuit board are known to crack. If the relay tests good outside the box but doesn't work in the box, the issue is the solder joint on the box itself. You can re-solder it for free if you have a soldering iron.

Also check if the AC fan works (turn on AC, the other fan should spin). The Civic has two fans — one for the radiator and one for the AC condenser. If the AC fan works, it proves the wiring harness is good and narrows it to the radiator fan circuit specifically.