How to Diagnose Radiator Fans Not Coming on

by Shayrgo Barazi

Radiator fans that don't come on pose a serious problem because this could cause overheating and inevitable engine damage if driven for long periods of time. Therefore, it is important not to drive the vehicle until the fans are operable again. Diagnosis is relatively straightforward and should only take a couple hours.

With the engine on and at operating temperature, disconnect the fan connector and measure the voltage with a multimeter. To do this, connect the red plug to one prong in the connector and the black plug to the other prong. You should see 12V. If you do, then there is something wrong with your fan and needs to be replaced. If you aren't getting 12 V, move on to the next step.

Measure the electrical resistance between the fan connector and the fan relay. You will need the vehicle's electrical schematic to know which prong to probe at the relay and connector. You should see 0 ohms. If there is continuity, move on to the next step. If there is no continuity, there is a break in the wiring between the plug and the relay and you should repair the wiring.

Replace the fan relay with a known good relay. There are other relays in the fuse box similar to the fan relay. Simply switch the relays to see if the fans operate. If they do, it was a bad relay. If they don't, you are going to have to continue back to the engine control unit to diagnose the issue.

Check electrical continuity between the relay and the engine control unit. Check the electrical schematic to see which pin on the ECU (Engine Control Unit) is for the engine fan. Connect the red plug at the relay and the other plug at the ECU. If you aren't getting continuity, fix the wiring between the ECU and the relay. If there is continuity, continue to the next step.

The ECU is most likely the culprit if you have gotten to this step. Try replacing with another and see if the fans work.


  • check Take your time and follow the steps exactly. To check for continuity between the relay and the ECU, you will have to get a long piece of wire and a friend to help you hold the wire at one end while you hold the other end.


  • close Watch your fingers while disconnecting the fan electrical plug because the fan could come on at any moment.

Items you will need


About the Author

Shayrgo Barazi is a college graduate with a degree in automotive engineering technology (B.S.c.) from Ferris State University. He is a successful writer and has taken a college level technical writing course. He currently works for Time Wave Media writing automotive DIY articles. He has an intuition for technology and has the capacity to write, too.

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