My Time Capsule occasionally goes to a yellow status and the fan runs fast

Just to add there is a secondary problem which is possible.. the temperature sensor on the power supply could be faulty. At the moment this is not amenable to DIY repairs.

The actual temperature of the TC is not actually over its limit.. but the sensor is giving false readings to the processor and fan controller.

Put your hand on the unit and feel if the Temperature at the top is hotter than expected.. you should also check the exhaust air underneath.. one side draws air in.. power inlet side.. and the opposite side the exhaust air flows out .. the temperature of exhaust should be at most a couple of deg warmer.

Check the fan speed and temperature in the airport utility diagnostic..

NB that Apple removed this function at Sierra (or H.S. not sure which).

The details are in a couple of threads

Airport is warning of overheating

Re: why is my airport extreme overheating?

As you can see this issue is reported fairly frequently..

Often it is a faulty or clogged fan. But the sensor .. particularly on the power supply is on a long lead relative to the other sensors and seems to go faulty for lots of people.

Let me point out a few items in this.

The Fan is PWM controlled.. So the min duty cycle is 57 which gives a speed of around 1500 rpm and is near silent.

When the device overheats (or gets wrong temp from sensor).. it increases the fan speed.. in this case as it tripped over it goes to its max duty cycle.. 255 and is now running at 5008 rpm.. which is noisy as blazes and gives the overheat warning.

If you look further you can see this is bad sensor.. Bad Sensor Mask has 4 in it.. Not a bad fan which is still 0.

If you check the temperatures down the list.

This does not give us readable to human numbers.. but if I look at the (internal firmware) to which I have access.. One of the numbers is nonsense.. -125. I am not at all sure how the diagnostics numbers relate to the actual numbers. If you compare to the good Time Capsule it is actually about the same with one of the numbers being several orders bigger than the others.

envstat

Current CritMax WarnMax WarnMin CritMin Unit

[accel0]

Accl X-Axis: -1005 0 0 0 0 none

Accl Y-Axis: 15 0 0 0 0 none

Accl Z-Axis: -11 0 0 0 0 none

Accl Temp: -1 0 0 0 0 none

[applfan0]

Fan_rpm: 5008 RPM

T_hdd: 0.000 degC

Fan_dcy: 255 0 0 0 0 none

[emc141x0]

T_internal: 24.625 degC

T_external1: -125.000 degC

T_external2: 27.875 degC

T_external3: 29.375 degC

I beat the problem briefly at least.. with some tricks.. like reversing the transistor sensor type.. then I tried grounding one side of the connector.. both of which worked for a time.. several months all together.. but now the fan is screaming again.. and I am trying to access the sensor chip.. to see if I can simply stop it reading that sensor.. I have got the SMB bus address and register but so far cannot write the change.. so more to come.

BTW.. Anyone with more digital electronics experience is very welcome to assist.. I am from the days of Valves.

Understand.. able!!

Just to complete my set of comments for anyone reading.

The info is still in High Sierra airport utility .. I have just installed HS on a new computer and have no trouble accessing data.

Apple called it a new name.. instead of Diagnostics..

It is now Analytics.

You must open the full configuration of the airport to see it.. ie click the icon of the airport and then click edit.. or hold down option key and double click the icon.

And has exactly the same info in it.

As you can see I have got the fan speed under control again.. i shorted the two wires of the power supply sensor and ran a 200ohm resistor to ground from that.. pretty simple fix in the end.. I spent ages working out how to defeat it..

It will take about a min or two from cold to bring the fan back under control.

The internal fan is very likely clogged with dust, so the cooling efficiency of the fan has been reduced by half or more. I've had the same issues with two Time Capsules that are about 18-24 months old. Blowing air at the opening at the bottom of the Time Capsule will do nothing to help a clogged fan, which is buried up near the top of the Time Capsule.

If the Time Capsule is still covered by the 1 year (in the U.S.) warranty, have Apple replace the device for you. If you have AppleCare in effect on one of your Macs, Apple will usually cover the Time Capsule under the AppleCare umbrella for up to 3 years from its purchase date. The "rules" are complicated, so get with Apple Support if the Time Capsule might qualify under this option.

If the replacement option is not possible, then you will need to consider opening up the Time Capsule to clean out the fan.....or better yet....replace the fan. The following links might help in that regard:

AirPort Time Capsule A1470 (Mini!) Teardown - iFixit

Genuine New Apple AirPort Time capsule A1470 Cooling Fan MG60121V1-C01U-S9A | eBay

If you decide to open up the Time Capsule, be extremely careful, as it is easy to break a connector or tear a ribbon cable in the process. Frankly, unless you are a dedicated do-it-yourself type of person, this task is better left to an experienced technician. I do not look forward to the possibility of having to open up my Time Capsules again.

Yeah.. Apple decided for you that you did NOT need to KNOW..

First test is easy one.. turn off your TC overnight and when you start it in the morning feel the temperature.. even better if you have kids thermometer .. insulate the bulb under some cotton wool and a piece of tape.. then check when the fan starts running hard.. how long it took to get hot and what the actual external temperature is hot.

You could also put the thermometer in the exhaust air flow from the TC and measure that.

Assuming you have iphone or iPad..

Try using the Airport utility for iOS as it still has diagnostics.

Which exact model is your TC and how old is it? Did you happen to recently update its firmware?

If you updated it to 7.7.9, then it would be an 802.11ac TC.

Looks like Bob got back to you before I did. I would second his comments and advice on next steps.