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Kevin Livingston

Technical Services Manager
North Georgia District

“FAIL TO PRESSURE” On A 48JC04-06 (Alert A321)

So, its Friday afternoon at 4:00 PM. You get a call of no cooling on a 48/50JC 04-06 Ultra High Efficiency Single Package Rooftop Unit with SystemVu™ 48JC04 – this system is one of Carrier’s high efficiency rooftop units. You open the system up and immediately go to the System Vu to check out the alarm. You see “Fail to Pressure”.

SystemVu Overview

Hold on, what is the SystemVu? Let’s check on this first. For the readers who aren’t familiar with System Vu, let’s do a brief review. This unit’s operation is controlled by the integrated SystemVu controller. It has been pre-configured with safeties, default settings and other logic to control all functions including the variable speed operation of the compressor, indoor fan motor and outdoor fan motor. It also controls the operation of factory options if they are provided. The unit can be hooked up to a conventional thermostat or Carrier sensor for operation with simply defining the sensor type mentioned in the manual. The factory default is Thermostat Control. In the manual are all the details to field change allowable settings to fit your application needs. For a quick start process if required, scan the QR Code below or enter the following URL.

*IMPORTANT: This literature is in HVACPartners or Carrier Service Tech App, it covers the 48/50JC 04-06 models with SystemVu controls version 3.X. The SystemVu controls is standard on all 48/50JC models. Several SystemVu articles are written in previous newsletters that you may find helpful.

Fail to Pressure Error

So now that we know what the System Vu is (more to come in future newsletters) and where to find out more about the system Vu, let’s get back to our original subject – “FAIL TO PRESSURE”. We know from our experience as HVAC technicians, that “Fail to Pressure” means the compressor is not pumping. We go to HVAC Partners or the app, look up 48/50JC 04-06 and go to controls. Scroll down to alarms. We read “Alert A321 – C. A1 FAIL TO PRESSURE: This alert occurs when 20 seconds after the compressor turns on, the suction did not drop more than suction amount (CIR.A MIN SUC.P) and discharge pressure did not rise more than discharge amount CIR.A MIN DIS.P). This alert causes a strike for the compressor. The alert resets automatically. The cause of the alert is usually compressor wiring causing reverse rotation or a faulty compressor.”   

We are thinking “Uh, Oh.” A bad compressor immediately comes to mind for many of us. Or, it could be a bad board. But more often, the board is not the issue.  It couldn’t be a reverse rotation issue. Or could it? Let’s explore.

Troubleshooting the Error Code

First, cut the electrical to the system off, tag out and lock it out. Safety first!  You can ohm out the compressor and check for good windings and ensure the compressor is not grounded, provided your volt/ohm meter is working properly and you have fresh new batteries in your meter.  Follow the fault in the literature. Once you have confirmed a good compressor electrically (hopefully), look back at the literature again. Follow the offerings in the literature under this fault:

“Alert A321: The cause of the alert is usually compressor wiring causing reverse rotation or a faulty compressor.”  

Begin asking questions to the owner such as:

  • Has anything happened recently to the building electrical wiring or system electrical wiring?
  • Has there been a recent thunder storm which knocked out electrical power in the area?
  • Have you called an electrician to do some work on your building electrical systems or panel box? Or was a main electrical transformer changed by the local power company in the area?”

Next, what does the literature state?  

“The cause of the alert is usually compressor wiring causing reverse rotation or a faulty compressor.”  

Now begin checking for reverse rotation of compressor which could happen with one of these episodes described above that could have caused a change of the 3-phase building wiring.  So, reverse the line voltage by switching any two leads of the 3-phase wiring of the system at the system.  Cycle the system back on, and your system begins operating. After a time delay, the compressor begins operating. Your suction pressure and head pressures are beginning to look normal. You have made the repair. You are the Friday afternoon hero! You repaired the system and it is now cooling down your customer’s building.

So, the next time you get a Friday Afternoon call on a 48JC system, don’t panic. Remember go to HVAC Partners. Look up the literature for this system, find your fault listed on the system Vu, and make the repair.

So, what we learned with this lesson:

  1. The SystemVu is common to 48JC systems
  2. You can work thru your control issues on the SystemVu Control
  3. You can find out and read your system faults on SystemVu
  4. Go to HVAC Partners to find the systems literature
  5. Most of the information you need will be located in HVAC Partners
  6. Safety first: Tag out, Lock out the disconnect to any system you are working on
  7. Always make sure your voltage/ohm meter has fresh new batteries before you start the day
  8. “Fail To Pressure”, means the compressor wiring causing reverse rotation or a faulty compressor
  9. Reversing any two wires on a 3-phase system will reverse rotation, if needed


  2. HVAC Partners: 48/50JC 04-06 Ultra High Efficiency Single Package Rooftop Units with SystemVu™ 48JC0; page 34
  3. Carrier Service Tech App

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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