Middleby Marshall Type K thermocouple
Use for Err 1 after confirming the probe length, connector, and controller terminals on the exact oven model.
$20-$120
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Middleby Marshall Oven Fault Codes - Quick Reference
Middleby Marshall conveyor ovens use digital controllers with error codes displayed on the front panel. Models include the PS528, PS536, WOW (Worlds Best Oven), and CTX series.
| Error Code | Meaning | Quick Fix |
|---|---|---|
| Err 1 | Thermocouple open or failed | Replace cavity thermocouple |
| Err 2 | High temperature limit tripped | Allow cooling, check setpoints |
| Err 3 | Ignition failure | Check gas supply and igniter |
| Err 4 | Air switch / blower fault | Check blower and air switch |
| Err 5 | Control board communication | Replace control board |
| Err 6 | Conveyor fault | Check drive motor and belt |
| HI LIM | High limit thermostat tripped | Allow oven to cool before reset |
| NO GAS | Gas valve did not open | Check gas supply and valve |
Most Common Faults
Err 1 - Thermocouple Fault
The cavity temperature sensor is open or failed. Middleby Marshall uses Type K thermocouples. Disconnect the thermocouple from the controller and measure resistance. An open thermocouple reads infinite resistance. Check the terminal block at the controller for corrosion. Replace the thermocouple if open or reading outside normal range.
Err 3 - Ignition Failure
Gas ignition did not occur during the timed ignition trial. Steps:
- Verify gas is on: check upstream shut-off and manual valve at oven
- Check gas pressure at manifold: natural gas 4-7 inches WC, propane 10-11 inches WC
- Listen for spark during ignition: if no spark, check igniter lead and ignition module
- Check flame sensor rod: clean with fine steel wool if coated with grease
- Check gas valve coil: should read 30-50 ohms
HI LIM - High Limit Tripped
The high limit thermostat (non-resettable or manual-reset type depending on model) has tripped. Allow the oven to cool completely before attempting reset. Find and correct the cause of the temperature excursion before returning to service. Check: setpoint on controller, cavity thermocouple accuracy, and insulation condition.
WOW Oven Notes
Middleby Marshall WOW ovens use dual-sided impingement (top and bottom). Both sets of jets must be clear for proper baking. Blocked jet fingers cause hot spots and can drive up oven temperature.
Parts Often Needed
| Part | Notes |
|---|---|
| Type K thermocouple | Most common part |
| Igniter / spark module | Replace if Err 3 persists |
| Conveyor drive motor | Replace on Err 6 |
| Gas valve | Replace if valve does not open |
Need Replacement Parts?
Before ordering, match the commercial oven part to the model number, electrical rating, connector, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Jump to Fix
- Err 1: Check thermocouple connector, measure resistance, replace the failed probe
- Err 3: Check gas, inspect igniter, test flame sensor, check gas valve
- HI LIM: Cool oven, identify the overheating cause, reset or replace the thermostat
When to Call a Pro
Middleby Marshall authorized service centers handle gas system repairs and control board replacement. For gas component work, use a licensed commercial kitchen technician.