Midmark M9 & M11 Error Code C983

If your Midmark Autoclave is giving you the error reading C983, here is everything you need to know about what is causing it ........and how you can fix it yourself






When power is supplied to the Main PC Board, current continuously flows thru the two ("normally closed") High-Limit Thermostats.

This circuit powers all line voltage components (except the Fan System). If either thermostat opens for any reason (overheat or malfunction) it triggers the error code C983. When this happens, the sterilizer will shut down until the unit cools, or the thermostat is replaced



When the High Limit Thermostat is Good, it is in a "closed" state. When it is defective, it is in an "open" state


Imagine the High Limit Thermostat as a gate in a fence. When the gate is "closed," the fence line is connected all the way through.....meaning the fence is "continuous" (has continuity). And in electricity, the current is allowed to follow the path to the next point

But when the gate is "open," the line of the fence is broken (not connected) meaning the fence is not continuous (does not have continuity). In electricity, the current stops at the gate opening

If your Midmark is displaying the error code C983, it means that your High Limit Thermostat is "Open" and will need to be replaced


The High Limit Thermostat is located on the bottom of the chamber. To get to it, drain all of the water from the reservoir and with the door closed, lay the Autoclave on it's side.

Locate the small access panel on the bottom of the sterilizer. Remove the access panel and you will see the 2 High Limit Thermostats
You just need to perform a simple "continuity" test. You will need a volt/Ohmmeter or Multi-meter to perform this test The setting you need will be in the "ohm" section of the meter where the Omega Symbol (Ω) is located. The setting is usually 2k or 200 (depending on the model of your meter).

If you have a digital meter and do not have these settings

• Lay the probes in such a way that the tips are touching each other
• Starting with the lowest number in the ohms range (Ω) turn the dial on the meter until the reading is "0.00"
• Now separate the tips of the probes and if the meter reading is "1" you have the right setting and are ready to test the High Limit Thermostat
• If the reading is anything but "1", turn the dial to the next number on the meter and try again
• Continue turning the dial until your meter reads "0.00" when the tips are touching, and "1" when they are apart

If you have an analog meter and do not have these settings

Repeat the test above for the digital meter, until the needle on the meter goes most of the way to the right ("0") when the probes are touching and stays to the left (infinity) when the probes are apart

With the Autoclave unplugged from the wall (Current Must Not Be Present when performing this test), the reservoir drained, and your Midmark Autoclave on it's side....remove the access panel from the bottom of the unit.

Note: This test must be performed when the thermostats are at room temperature. If the thermostats are hot, let them cool down before proceeding with this test

• Test One Thermostat at a time
• Tag each of the two wires so you know which wire goes where, and then remove both wires from the thermostat.
• With your meter set on the proper setting as described above, place a probe on each terminal on the thermostat (it doesn't matter which color probe goes where)
• Check your reading
• If your meter reads 0 (or close to it) (the needle is all the way to the right on an analog meter) then there is continuity, and the High Limit Thermostat is good
• If your meter reading is 1 (all the way to the left on an anaolog meter), then the High Limit Thermostat is bad and needs to be replaced

Be sure to test both High Limit Thermostats. It is possible they are both defective


If they both check out as being in a "closed" state, then you will need to replace them both. I know this probably doesn't make a lot of sense to you, but let me explain.

The High limit Thermostat is factory set to "open" (break contact and stop the flow of electricity) at a high temperature (450°F).

What it means if they both check out as being closed is that one (or possibly both) of the High Limit Thermostats is breaking the connection at a lower temperature than the factory setting. This stops the electricity from moving on to the next component, and causes the error code C983,

When this happens, determining which one is defective is beyond the scope of what we are doing here (If you really want to know, then you need to buy Jestine's course (you can find it here))

So, you will need to replace both of them. The two High Limit Thermostats are identical, so order 2.


You have already done the hard part (troubleshooting). The actual repair is simple.......just replace the defective High Limit Thermostat with a new one.

We recommend you wait until you have the new one in hand before removing the defective one.....so you can simply reconnect the wires to the new, as you take them off the old. This makes the repair simple, fast & easy to do

How Much Is Your Time Worth?

The average charge for an onsite technician to make this repair is in the $250 to $300 range (or even more), plus the cost of the high limit thermostat(s).

And if you were to ship it off to the factory, you will be charged more than $1100, plus the cost of shipping (in both directions).

In reality, replacing the High Limit Thermostat(s) will take you only a few minutes to complete