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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

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  • 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
4.7 / 5.0
13 Reviews
5 Stars
4 Stars
3 Stars
2 Stars
1 Stars
Works great so far. The instructions are not the best so set-up is a little confusing but the product itself has been awesome so far
April 17, 2018
1 month ago
Works now
The unit is now working, but thankfully I opened the front face to wall mount it. The wiring in the terminal block was not properly torqued down allowing the wiring to easily pop free. The loose connection could have caused more significant issues. Since I had it opened and needed to fix it I tinned all of the stranded wires to ensure proper contact with the set screws. Another option would be to use crimped on ferrules.
March 3, 2018
2 months ago
Johnson A421
After my A419 lost its mind, I had to get a new controller. This new model is great, easier to use and the backlight is very nice.
December 31, 2017
4 months ago
Control your temper/ature
Great device. I turned an old commercial beer fridge into a produce cooler and on the advice of a beer-maker friend I got this unit to change the temp control to a slightly higher temp for the produce. Works great, easy to program.
November 27, 2017
6 months ago
Works like a charm
The control box works just like it says it does. Its easy to use and holds a constant temperature like a pro. I would definitely buy this again and have absolutely no problem recommending this anybody, it's worth the money.
October 4, 2017
7 months ago
Seems to be working fine. Directions difficult to understand. I mounted to the side of a 22 cuft chest freezer and it is keeping right on the temperature that I set.
July 8, 2017
10 months ago
Great product
Easy to set up and get operating. I'd recommend it to anyone lagering beer. Truly fresh Octoberfest ready by mid September
July 4, 2017
10 months ago
A421 temp. controller
I got this unit because I live in South Florida where the temperature in the house or garage is always over 75. I have a small refrigerator and with it I can brew beer at any outside temp. It easily was set to maintain a temp between 60 and 70 degrees.
March 30, 2017
1 year ago
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 year ago
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 year ago
Start typing your question and we'll check if it was already asked and answered. Learn More
Browse 11 questions Browse 11 questions and 34 answers
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
Does this device work with mini fridges?
A shopper on Sep 10, 2017
BEST ANSWER: Yes - I use this device to control my converted mini fridge kegerator. The controller interface is not necessarily user friendly to those not familiar with PLC controllers, but there are enough videos out there to show the novice how to get it set up. My temperature controller holds 38 deg F +/- 2 deg F all day long.
Can this be used on a window mount a.c. unit?
A shopper on Aug 5, 2017
BEST ANSWER: You could, but I don't know how satisfactory it would be. The sensor cord is not real long and with a window a.c. the air coming out tends to stay close to the same temp all the time so you would really want to keep the probe well away from the ac unit. I guess you could rig up sometbing with a couple heavy duty extension cords to get the u it and the probe far enough from the a.c.. Also need to check to make sure watts the ac draws doesn't ecceed the watts the unit can support.
For lagering 10 gallons, how wide of a temperature interval do people normally use? I understand that the wider the range, the less the unit turns off and on and saves wear and tear. I've been setting the shut-off and turn-on temperatures three degrees above and below the target temperature (6 degree range). Appropriate?
A shopper on Mar 27, 2018
BEST ANSWER: I have my range set for a 4 degree variance. I don't want it to go over 68 degrees. It works perfectly. But you do need to set the thermostat so that the freezer won't chill for too long because it will cool too quickly for the controller to respond. For example, when the controller senses a temperature over the upper limit, the freezer gets turned on. It takes time to register for the lower limit to be sensed and then the freezer temperature is lower than the lower limit.
I would like to maintain a 28 deg temp. Would it be best to use this with a fridge or a freezer? Is there any reason that would not work?
A shopper on Aug 23, 2017
BEST ANSWER: For that low temp, if Fahrenheit, use a freezer. I have one for my fridge that I use for controlled fermentation. Then I have one on a deep freeze I use for my kegged beer.
Can I run 2 heating pads at the same time?
John F on Jun 5, 2017
BEST ANSWER: I'm not totally sure. I use mine for controlling temp in a refrigerator used for primary fermentation of wine. It works really good. I would think two heating pads would not have the amperage load of one fridge.
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.
Can I use this on a window mount a.c. unit?
Tyler Z on Aug 5, 2017
BEST ANSWER: I've not done that but a quick go ogle search indicates it has been done. I assume you are thinking about designing some type of cold room\fermentarium. That is what I found.
Can I use this to run 2 heating mats at the same time?
John F on Jun 5, 2017
BEST ANSWER: I don't think so, it comes with one temp probe.

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