Johnson A421 Digital Temperature Controller


SKU# 41944

Control the Temperature. Control the Flavor. Just Plug and Play.

Flat Rate Shipping only $7.99 only from Northern Brewer
The Digital Temp Controller operates by overriding the internal thermostat in a refrigerator or freezer and gives the brewer greater control over the fermentation. Precise control of fermentation temperature is one of the most beneficial things you can do to improve your home brew beer.

It is also ideal for controlling the serving temperature of your kegerator or keezer.

Availability: In stock

Digital Temperature Controller

More Views

  • Johnson A421 Digital Temperature Controller


Product Details

The digital controller has a digital display with adjustable differential. It can be used to heat or cool a fermentation environment. When hooked up to a refrigerator, it turns the compressor on when the ambient temperature rises above your programmed temperature; when hooked up to a heater, it activates when the temperature drops too low. A sensor probe monitors the temperature inside while the readout displays it in either Fahrenheit or Celsius. This is the ideal choice for lager brewers who include a diacetyl rest in their fermentations or for ales like altbier or Kolsch that require fermentation temperatures in the low end of the ale range.

External overriding thermostat with backlit digital display. High and Low Temperature Stop Values determine whether the unit is heating or cooling. There is no longer a need to open the unit to switch between heating and cooling modes.


  • Plug 'n play - this controller is pre-wired, so you don't need a degree in electrical engineering to get it up and running.
  • New! Backlit display for even easier-to-read display - this one speaks for itself, but when we say easy, we mean easy. 
  • New! Faster Programming with three-button interface
  • New! High and Low Temperature Stop Values determine whether the unit is heating or cooling.
  • New! Internal defrost settings
  • Fully detailed manual, excellent service and support
  • Features a wide setpoint temperature range (-40° F to 212° F). The temperature sensor is a 1.94 inch thermistor on an 8 foot cable.
Additional Information
4.8 / 5.0
5 Reviews
5 Stars
4 Stars
3 Stars
2 Stars
1 Star
Only had a month but so far so good. It keeps my fridge at 62 F and the mead is fermenting very well.
January 30, 2017
1 month ago
Works better than expected
I was expecting to have to wire up the thermostat to my A/C glycol chiller. Not so. Just plugged the chiller into the receptacle on the thermostat, set the temps, and away it goes. I would buy this again.
August 28, 2016
Johnson A421 Digital Temperature Controller
Awesome piece of equipment, works as expected and isn't too difficult to program.
August 2, 2016
Easy install
Awesome product. I already use the older 419 and it is trouble free. 421 even easier to install for my kegerator.
February 7, 2017
1 month ago
Easy to use, works great!
Ordered this control so I could use my old freezer for lagering. First batch of pilsner is going great - the freezer maintains the temperature specified in the recipe just fine and it's easy to adjust the temperature as needed.
August 9, 2016
Start typing your question and we'll check if it was already asked and answered. Learn More
Browse 4 questions Browse 4 questions and 7 answers
The product details for this indicate that it has an 'internal defrost setting'. Does this mean that the controller is able to be used in freezing conditions and that the screen will not fail due to cold?
A shopper on Feb 7, 2017
BEST ANSWER: This feature may not prove very helpful when using it for a fermentation. The unit can be programmed with a defrost cycle that turns the fridge/freezer off in specific internals/periods to allow frost to melt and avoid it building up inside the fridge/freezer.
Does this have a Ssr relay?
Mark V on Jan 18, 2017
BEST ANSWER: This description is right out of the user manual:
The A421 Series controls are single stage, electronic
temperature controls with a single-pole, double-throw
(SPDT) output relay. The controls feature an adjustable
backlit LCD for viewing the temperature and status of
other functions, and a three-button touchpad for setup
and adjustment. An LED indicates the On/Off status of
the output relay.
how can I see installation instructions? I want to check compatibility with my refrigerator prior to purchasing temperature control.
Steve on Jul 27, 2016
BEST ANSWER: howdy. I just bought this a month ago. it works perfectly as advertised. This device will work with pretty much any type, size, manufacturer of refrigerators out there. all you do is plug your current fridge into Johnson A421 Digital Temperature Controller power strip?. the way it works is easy. you plug your fridge into the temp controller, plug the temp controller into the power outlet, and then input your desired temperature range into the Temp Controller digital interface. i had it setup in 30 minutes, including mounting, etc. hope that helps.
I am considering using this on a side by side refrigerator If using on the refrigerator side, how will this then impact the freezer side
D O on Aug 13, 2016
BEST ANSWER: This sort of controller functions by turning off the power to the entire refrigerator, so when it shuts off power to the refrigerator side, it will be also shutting off the freezer side of the unit. You would not want to have frozen food or other items in the freezer, as it would not always be holding them at freezing temperatures. -Mike W, Northern Brewer

You may also be interested in:

Please wait...

Your selected product has been added to your Cart.
Add Some Bottles to Your Order Today and Take 10% Off!

Continue shopping
View cart & checkout