Commercial Dishwasher Repair: Commercial Dishwasher Repair: Common Faults and Field Fixes

Commercial dishwashers are among the most heavily used pieces of equipment in any food service operation. A high-volume restaurant may run 200–400 racks per day through a conveyor machine. When the dishwasher goes down, the entire back-of-house operation backs up.

Unlike residential dishwashers, commercial units operate at higher temperatures, use chemical sanitizers instead of heat drying, and are built for continuous operation. They also fail in distinct, diagnosable ways. Here's how to work through the most common faults.

Before diagnosing, identify the machine type — each has different systems and failure modes:

• Undercounter (door-type): Single-rack machines common in bars and small restaurants. Similar in layout to a residential unit but built for commercial duty.
• Conveyor (flight-type or rack conveyor): High-volume machines that move racks or dishes continuously through wash, rinse, and sanitize zones.
• Glasswasher: Specialized undercounter units for bar glassware, operating at lower temperatures with gentler wash action.
• Pot washer: Heavy-duty units for pots, pans, and sheet trays.

This is the most common complaint and has multiple possible causes.

Wash arm inspection: Clogged wash arm nozzles are the leading cause of poor wash results. Remove the wash arms (usually a quarter-turn or single bolt) and flush them under running water. Use a toothpick or fine wire to clear individual nozzles. Inspect for cracks or warping that affect spray pattern.

Wash tank temperature: Most commercial dishwashers require 150–160°F wash water. Use a calibrated thermometer to verify — a tank that's running cool won't activate the detergent properly. Check the tank heating element and thermostat if temperature is low.

Detergent concentration: Low detergent concentration produces poor results even with correct temperature and pressure. Check the chemical dispenser pump — test output volume against the manufacturer's spec. Inspect the injection point for blockage.

Water pressure: Low incoming water pressure reduces wash arm spray force. Most machines require 20–25 PSI at the inlet. Check the supply valve and inlet screen.

Rack loading: Not a mechanical fault, but worth noting — improperly loaded racks block spray coverage. Confirm staff are loading per the manufacturer's guidelines.

High-temperature machines sanitize with a final rinse at 180°F. If the sanitizing rinse isn't reaching temperature, the machine is a health code violation.

Booster heater: Most high-temp machines use a booster heater to raise incoming water from ~120°F to 180°F for the final rinse. Test the booster heater element for continuity and correct resistance. Verify the thermostat is set correctly and functioning.

Final rinse temperature gauge: Confirm the gauge is accurate with a calibrated thermometer. A faulty gauge may show 180°F when the actual temperature is lower.

Rinse solenoid valve: The valve that admits hot water for the final rinse can stick or fail. Test the solenoid coil for correct resistance and verify the valve opens fully.

Not filling:

• Check the water inlet solenoid valve — test coil resistance and verify voltage is present when the fill cycle is active
• Inspect the inlet screen for scale or debris
• Verify incoming water pressure is adequate

Overflowing:

• The float switch or pressure switch that signals "tank full" has failed — test for correct operation
• The inlet solenoid valve is stuck open — replace the valve

Commercial dishwashers have door interlock switches that prevent operation with the door open. A failed door switch is a common cause of a machine that powers on but won't run a cycle.

Test the door switch for continuity with the door closed. On conveyor machines, inspect the curtain switches and end-of-conveyor sensors that serve the same safety function.

Also check the control panel for fault codes or indicator lights that point to a specific interlock failure.

Wash pump noise: A grinding or cavitating sound from the wash pump usually indicates debris in the pump impeller. Remove and inspect the pump — broken glass, bone fragments, and utensils are common culprits in commercial kitchens.

Conveyor noise (conveyor machines): Worn conveyor drive chain, misaligned sprockets, or a failing drive motor. Inspect the chain tension and lubrication; check drive motor amperage draw.

Vibration: Loose wash arm, worn pump bearing, or an unbalanced blower wheel on drying systems. Identify the source by isolating components during operation.

Chemical dispensers (detergent and rinse aid) are often overlooked but critical to machine performance.

• Check the chemical supply containers — empty containers are the most common cause of dispenser failure
• Inspect the dispenser pump tubing for cracks, kinks, or air leaks
• Test pump output volume against spec
• On peristaltic pumps, inspect the pump rotor and tubing for wear
• Verify the dispenser is receiving the correct trigger signal from the machine controller

In hard water areas, mineral scale accumulates on heating elements, wash arms, and tank walls. Scale acts as an insulator on heating elements, causing them to overheat and fail prematurely. It also clogs wash arm nozzles and reduces pump efficiency.

Establish a deliming schedule based on local water hardness — typically monthly in hard water areas. Use a commercial descaler approved for the machine type and follow the manufacturer's procedure exactly. Never use acid-based descalers on aluminum components.

• Daily: Clean wash arms, clear debris from tank strainer, wipe door gaskets
• Weekly: Inspect chemical dispenser output, check wash and rinse temperatures, clean curtains (conveyor machines)
• Monthly: Delime tank and heating elements, inspect door gaskets and hinges, check all solenoid valves for leaks
• Quarterly: Full inspection of pump, motor, and conveyor drive; test all safety interlocks; verify chemical concentrations

Commercial dishwasher service draws on electrical troubleshooting, plumbing, and chemical systems knowledge — skills that transfer directly to other equipment categories.

Metro Star Mechanical's training program covers the full range of commercial kitchen equipment, including warewashing systems. View our courses or contact us to learn more about hands-on training in Atlanta.