Someone brought me a 03 Mazda 3 to look at and while checking some unrelated things I noticed that the radiator fan was cycling on 15 seconds and off 15 seconds constantly, even though the ECT was reading a constant 88°C.

That just didn't seem right to me so I thought I'd ask if that's normal for this car?

The unrelated issues are that the car needs a new CAT ( P0420 verified ) and it's running a bit lean, LTFT of 14%, most likely a vacuum leak since it goes down to 5% at 3000 rpm.


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You can see that the temp stays flat all the time, even while the fan cycles on and off ( you can see where that happens by the MAF signal ).

Just seems a bit odd to me. A/C is not on, engine is warm after a 30 minute drive in the city. I'm wondering if maybe the ECT is reading accurately...

Unfortunately, I don't have access to the car for further testing.

  • The regularity of it does seem suspect. 88°C should be in the ballpark of what temperature it should be, but a constant temp would bother me. Is this even at first start? If so, check the coolant temperature sensor to see if it's good. If it was maintaining a constant 88°C actual, I don't think the fans would turn on at all, as I think this would fall under the threshold. Was the AC on while you were looking at it? That would regularly cycle the fans. The other thing is, fix what you know is bad first ... it may surprise you. May 30, 2018 at 9:23
  • Is the A/C on? - that can put the fan on more often...
    – Solar Mike
    May 30, 2018 at 9:27
  • @Pᴀᴜʟsᴛᴇʀ2 works in programming : fix the first error and 90% are sorted...
    – Solar Mike
    May 30, 2018 at 9:45
  • @Pᴀᴜʟsᴛᴇʀ2 See my edit. May 30, 2018 at 10:11
  • Test the ECT sensor first, then go from there. May 30, 2018 at 10:31


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