Rooftop Package Unit Not Cooling: What a Tech Checks on a Commercial Call

Most commercial rooftop package units stop cooling for one of four reasons: low refrigerant, a dirty condenser coil, a failed compressor, or a control/electrical fault. The hard part is figuring out which one, because the symptoms overlap. Here’s what a tech actually checks on a commercial RTU call, and what that tells you before you pick up the phone.

What’s Actually in a Rooftop Package Unit

A packaged RTU has everything in one cabinet: the compressor, condenser coil, condenser fan, evaporator coil, blower, and controls. That’s convenient until something breaks, because all that equipment is on a roof exposed to sun, rain, birds, and grease exhaust from kitchens below. Commercial units work harder than residential equipment, often running 12 or more hours a day in a building that doesn’t forgive a hot afternoon.

The Most Likely Causes, in Order

Dirty condenser coil. The condenser coil on a rooftop unit gets coated with dust, cottonwood, grease, and debris. When the coil can’t reject heat properly, head pressure climbs, the compressor runs hot, and cooling capacity drops. Sometimes the unit trips out on a high-pressure lockout. A tech will shine a light through the coil fins. If you can’t see daylight through them, that’s your answer. Cleaning usually brings the unit back quickly, but it’s a professional job on commercial equipment — requires the right coil cleaner, proper rinsing, and care not to damage the fins.

Refrigerant leak. Refrigerant doesn’t get “used up,” so if the charge is low, there’s a leak somewhere. A tech will connect gauges and check suction and discharge pressures. Low suction pressure with warm supply air is a strong indicator. Before adding refrigerant, the leak needs to be found and repaired, otherwise you’re just buying time.

Electrical or control fault. This covers a wide range: a tripped contactor, a failed capacitor on the condenser fan motor, a blown fuse in the control circuit, or a faulty thermostat signal. A tech will check voltage at the compressor contactor, test capacitors, and verify the thermostat is actually sending a cooling call. These components sit next to live high-voltage wiring on a commercial unit — a tech diagnoses and replaces them, not a building manager.

Compressor failure. This one hurts. The compressor is the most expensive component in the unit. A failed compressor often shows equalized or nearly equalized suction and discharge pressures, meaning the pressure differential collapses. If the compressor isn’t energizing at all, the issue may be electrical upstream of it rather than the compressor itself. A completely failed compressor on an older unit often means a replacement decision rather than a repair. If the unit is 15 or more years old and the compressor is gone, the economics usually favor a new unit.

Airflow problems. Less dramatic but worth mentioning: a clogged filter, failed supply fan motor, or blocked return grille can make it seem like the RTU isn’t cooling when it’s actually running fine. If supply air is cold but certain zones are warm, the problem may be in the duct system or dampers, not the RTU itself.

How a Tech Works Through the Diagnosis

A good tech starts at the thermostat, confirms there’s a valid cooling call, then goes to the unit. They’ll check:

1. Are the condenser fans running? Are the indoor blowers running? (Verify airflow before pulling gauges.)
2. What are the suction and discharge pressures?
3. Is the compressor energized? What’s the amp draw?
4. What’s the supply air temperature at the unit?
5. Any lockout codes on the controls? (Most commercial controls log fault history.)

That sequence usually points to the problem within 20 to 30 minutes. If the pressures are normal, the compressor is running, and supply air is cold, the tech starts looking at airflow or controls. If pressures are off, the refrigerant circuit gets more scrutiny.

Fault codes on commercial units vary by manufacturer. If a tech calls out a lockout code, ask them to show you where it’s displayed on the unit. It should be visible on the board or the display. Any reputable tech can explain what a code means in plain language.

What You Can Safely Check Before Calling

There are a few things a property manager can look at without touching the equipment:

• Filters. If they’re packed solid, change them or pull them temporarily to see if airflow improves. A building with no maintenance history sometimes has filters that haven’t been touched in a year.
• Thermostat settings. Sounds obvious, but verify the setpoint, mode (cool, not fan-only), and that the schedule hasn’t been changed. A night setback that someone altered can look like an equipment problem.
• Breakers. Check the main disconnect at the unit and the breaker panel. RTUs often have a local disconnect on the roof and a breaker in the electrical room. Both need to be on.
• Condensate drain. If the unit is cooling but dumping water inside, a clogged condensate drain can trigger a float switch that shuts cooling off. That’s a maintenance item you can address.

Everything past those four items, refrigerant pressures, capacitors, contactors, wiring, coil cleaning, belongs to a licensed tech. Getting it wrong costs more than the service call.

Call Us

If filters are clean, breakers are on, thermostat is correct, and the unit still isn’t cooling, you need someone on the roof with gauges and commercial RTU experience. Don’t let a residential-only tech guess their way through large commercial equipment.

We work on commercial RTUs across the Bay Area. We can also tell you straight whether it’s worth repairing or time to replace — that conversation matters before you spend money on a compressor in a 15-year-old unit.

Call (925) 999-4095 or book at bayareahvacservice.com. We’ll get you on the schedule quickly, often same or next day when we can.

Key takeaways

• Dirty condenser coil, low refrigerant, electrical faults, and compressor failure are the four most common causes of an RTU not cooling. A dirty coil may be the single most frequent culprit on commercial equipment.
• A tech verifies fans and airflow first, then pulls gauges on the refrigerant circuit. Pressures and the pressure differential across the compressor together usually narrow the problem fast.
• Property managers can safely check filters, thermostat settings, and breakers before calling. Refrigerant and high-voltage work require a licensed tech.
• On units over 15 years old, a failed compressor often makes replacement more economical than repair. Get that conversation before authorizing a compressor job.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

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• Mitsubishi Mini-Split Blinking a Fault Light? What It Means and What to Do , A blinking light on your Mitsubishi indoor head is the system telling you something is wrong. Here is what the blink pattern means, the checks you can safely do yourself, and when to bring in a factory-trained tech.
• Why Your Mitsubishi Mini-Split Is Leaking Water Inside (and How to Fix It) , A Mitsubishi mini-split dripping water inside almost always comes down to condensate that can't drain. Here's what causes it, the safe checks you can do yourself, and when to call a tech.