Hobart E-04 Heating / Temperature Sensor Error. Complete Fix Guide

Error Code: Hobart E-04

What it means: The control board detected a temperature sensor reading outside the expected operating range. The thermistor in the wash tank or booster heater reports a temperature that is too high, too low, or changing too fast to be physically possible. The board flashes E-04 and disables the heaters.

E-04 is a heating system malfunction code. It does not mean the water is too hot or too cold in normal operation. It means the sensor tells the board a value that does not make sense. A thermistor shorted to ground might report 300 degrees. An open thermistor might report well below freezing. In both cases, the board recognizes the reading as invalid and faults out.

This code appears on Hobart AM-14, AM-15, CLe, and CX-series machines. On three-phase models, E-04 can also indicate a heater element shorted to ground on one leg, which the control board interprets as an impossible current draw.

The E-04 error can also appear after a routine cleaning. Water intrusion into the thermistor connector, or a cleaning chemical left on the sensor probe, can produce erratic readings that trigger the fault.

Common Causes

• Failed temperature sensor (thermistor). The primary cause. Hobart uses NTC (negative temperature coefficient) thermistors. Resistance decreases as temperature rises. At 77 degrees Fahrenheit, a good NTC thermistor reads approximately 10,000 ohms. At 140 degrees, the reading drops to approximately 1,000 to 1,500 ohms. A shorted thermistor reads near zero ohms. An open thermistor reads infinite ohms. Both trigger E-04.
• Loose or corroded thermistor connector. The connector between the thermistor and the control board sits in a damp environment. Moisture and detergent residue creep into the connector pins. This adds resistance to the circuit or bridges pins. The board sees a voltage it cannot interpret.
• Booster heater element burned out or shorted to ground. On conveyor and flight-type dishwashers with a gas or electric booster heater, the heating elements in the booster tank eventually fail. Scale buildup on the element surface traps heat, the element overheats internally, and the sheath cracks. An element shorted to chassis ground can blow a breaker or trip a GFCI, producing E-04.
• Scale buildup on the sensor or heater. Hard water areas produce rapid scale accumulation. A thermistor encased in calcium deposits reads the temperature of the scale, not the water. The water inside the tank may be at 130 degrees, but the scale-insulated sensor reports 110 degrees. The board sees slow heating, extends the heat cycle, and eventually faults out with E-04 when the preset heat time elapses.
• Failed contactor. The heater contactor supplies line voltage to the heating element. The contactor coil can fail and not pull in, or the contacts can weld shut. Welded contacts keep the heater running after the board removes the call for heat. The water overheats, the high-limit trips, and the board registers E-04 from the overshoot.
• Water flow issues. On models where the temperature sensor sits in the booster tank, insufficient water flow through the tank causes the heater to cycle rapidly on a small volume of water. The sensor sees erratic temperature swings. A stuck float switch, closed fill valve, or plugged inlet screen reduces flow. The heater fires into a partially empty tank, the sensor sputters between 160 and 212 degrees, and the board logs E-04.
• Control board failure. Less common, but the board can lose the reference voltage for the thermistor circuit. If the board provides 5VDC to the thermistor input but the internal voltage regulator drifts, every sensor reading becomes invalid. This usually produces E-04 regardless of what sensor is connected.
• Three-phase electrical issues. On three-phase machines, if one leg of the supply loses power, the heating element receives reduced voltage. The heater draws less current and produces less heat. The wash tank takes too long to come up to temperature. The board detects the extended heat time and faults with E-04. Check all three phases with a multimeter before replacing any parts.
• E-04 appearing after cleaning. If E-04 shows up immediately after a deep clean or descaling cycle, suspect water in the thermistor connector. Cleaning crews sometimes spray water directly at the sensor or use descaling chemicals that attack the sensor sheath. Let the machine dry out for 24 hours with the access panel off. If the code returns, replace the thermistor.

Step-by-Step Fix

1. Record the temperature reading on the display. Before turning anything off, check the machine display for the reported wash tank temperature. If it flashes a number like 220 degrees or 32 degrees alongside E-04, the thermistor is likely bad. A reading near room temperature when the machine is cold but still showing E-04 suggests an open or shorted sensor.

2. Test the thermistor. Disconnect power. Locate the temperature sensor in the wash tank or booster tank. On AM-14 machines, it is a probe with two wires that screws into the side of the wash tank. Unplug the connector from the control board. Set your multimeter to resistance mode. Measure across the two thermistor wires at the connector. At room temperature (70 to 80 degrees), you should see approximately 8,000 to 12,000 ohms. Heat the sensor probe with a hair dryer. The resistance should drop smoothly as the temperature rises. If the reading is OL (open) or zero (shorted), replace the thermistor.

3. Check the thermistor wiring and connector. Large-scale voltage drop across a connector produces bad readings. Inspect the connector pins for corrosion. Clean with electrical contact cleaner and a small brush. Reconnect and retest. If the connections look good but the reading is erratic, wiggle the wires near the connector while watching the multimeter. A jumpy reading points to a broken wire inside the insulation.

4. Inspect the heater element for scale and damage. Remove the wash tank access cover. Look at the heating element. White or gray hard scale on the element surface indicates hard water buildup. Scale acts as insulation. The element produces heat but cannot transfer it to the water. The sensor sees slower-than-expected temperature rise. Descale the tank with a commercial deliming solution. Rinse thoroughly. If the element sheath is cracked, bulging, or has holes, replace the element immediately.

5. Test the heater element resistance and ground continuity. With power off, disconnect the heater element wires. Measure resistance between the two element terminals. A good element typically reads 9 to 12 ohms for a 4.5 kW element at 208V. Compare with the element rating on the machine data plate. Then test for ground short. Set your meter to the highest resistance scale. Touch one lead to an element terminal and the other lead to a clean metal ground on the machine chassis. The reading should be OL (open). Any reading below 1 megohm indicates a deteriorating element that will soon short to ground. Such an element can trip breakers and produce E-04.

6. Check the heater contactor. Turn power back on and start a cycle that calls for heat. Listen for the contactor to click in. If no click, measure voltage at the contactor coil. You should see the control voltage (usually 24VAC or 120V depending on the model) across the coil terminals. Voltage present with no click means the coil is open. Replace the contactor. If the contactor clicks but the heater does not come on, check the contacts. With power off, measure resistance across the load terminals of the contactor. An open reading means the contacts are burned out.

7. Verify water flow to the booster heater. On machines with a separate booster heater, check that water is flowing through the booster tank. Open the booster drain and see if water flows freely. Slow flow means the inlet filter or fill valve is restricted. Clean the inlet screen. Verify the float switch moves freely and the fill valve opens fully.

8. Check the incoming power supply. On three-phase machines, measure voltage from each phase to ground and between each pair of phases. A lost phase on a 208V system leaves only 120V across the element. The heater runs at roughly one-third of its rated power. The tank takes too long to heat, and the board faults with E-04. Three-phase power issues require an electrician. Do not attempt to reconnect phase wiring yourself.

9. Replace the thermistor and retest. If all the above checks pass and the thermistor readings were borderline (resistance slightly out of spec but not open or shorted), replace the thermistor anyway. NTC thermistors drift over time. A $30 sensor replacement solves a lot of intermittent E-04 faults that no individual test catches.

10. Cycle power to clear the error. Turn the main breaker off and wait one minute. Turn it back on. The E-04 should be gone. Run a full wash cycle and monitor the temperature for steady rise. If the code returns, the control board may need replacement.

Hobart Error Code Reference Table

When to Call a Professional

Call a commercial kitchen technician if any of these apply:

The thermistor tests good, all wiring checks out, the heater element resistance is correct with no ground shorts, and the E-04 still returns. The control board may need a factory-authorized replacement with proper programming.

The heater element has cracked and leaked water onto live electrical connections. A shorted element can weld contacts or damage the control board. Let a technician evaluate the full electrical system before replacing parts.

You find a lost phase on a three-phase machine. Electrical supply issues at the breaker panel need a licensed electrician. Do not open the panel yourself.

Scale buildup has encased the heater element in a thick mineral crust that you cannot remove with deliming chemicals. The element may need to be cut out and replaced. Hard water also scales the inside of the tank, float chamber, and fill valve. A full descale and replacement of all affected parts is a multi-hour job best handled by a professional.

The E-04 appeared after a cleaning or descaling procedure and persists after drying out the sensor connector. The descaling chemical may have damaged the sensor sheath or the control board input circuit. A technician can test the board voltage reference and confirm whether the board is salvageable.

Technician tip: When you call for service, keep the machine on and the E-04 displayed. The technician wants to see the exact code and the reported temperature on the display. Turning it off before they arrive erases diagnostic information. Tell them “E-04, wash tank thermistor reads [your measured ohms], heater element is [your measured ohms], resistance to ground is [your measurement].” This cuts their diagnosis time in half.