Middleby Marshall Pizza Oven Alert High Temp Error: Sensor or PLC

What does Middleby Marshall High Temp Error mean?

The controller of a PS640G or PS670G holds the chamber at its setpoint by reading a thermocouple and switching the heat. High Temp Error is shown when the thermocouple or the PLC, the programmable logic controller that makes the decisions, is malfunctioning. In that situation the controller cannot rely on its temperature reading or on its own logic, so it cannot safely hold the chamber temperature. Rather than guess, the oven stops, and the manual says to turn it off and call for service. The reasons behind the alert are the thermocouple and its connections, damage to the thermocouple wiring, the PLC and its inputs, or the heat controls that respond to the PLC. A thermocouple that fails open reads as an extreme value, and wiring damaged by heat or a catch on a pan does the same. A controller input that has failed gives a false reading, and a heat control that does not respond leaves the burner on. Because a wrong reading can lead to genuine overheating, the oven should not be baked through while the alert is showing.

Affected models and control systems

This applies to Middleby Marshall PS640 and PS670 conveyor ovens with the touch-screen controller, such as the PS640G and PS670G. Other controller generations use different temperature alerts.

Code numbers and their meaning can change between control generations. We match the full model and control before acting on a code.

What you may notice

• High Temp Error on the touch screen
• Oven shutting down or heating stopping
• Chamber temperature on the screen that looks wrong
• Pizzas baking too dark or too pale

Possible causes

Listed in Middleby Marshall's documentation

• The thermocouple and its connections
• Thermocouple wiring for damage
• The PLC and its inputs
• Heat controls that respond to the PLC

Other likely causes — what usually sits behind High Temp Error on this control. A technician confirms which one it is before any part is replaced.

• Thermocouple tip corroded or burned after years at high heat
• Wiring pinched where it passes the oven frame
• PLC input that drifts and reads far from the real temperature

Safe checks before calling for service

How a technician diagnoses High Temp Error

1. Read the alert and the temperature shown.
2. Compare the screen reading with a reference thermometer in the chamber.
3. Check the thermocouple, its connection and wiring for damage.
4. Measure the thermocouple output with a meter.
5. Check the PLC input and the heat control response.
6. Replace the thermocouple or PLC part only after the test.

How High Temp Error differs from similar codes

ALM 1 on older PS520 and PS528 controllers is a fixed 650 F high limit, whereas High Temp Error here points to a sensor or controller malfunction.

Technical reference

Document

PS640G I&O Manual 60251 Rev J, screen alerts

Edition

P/N 60251 rev J, (c) 2023 Middleby Marshall Inc.

Models / control

Middleby Marshall PS640 / PS670 touch-screen controller (screen alert)

Section

Section 3 Operation - Quick Reference: Troubleshooting, Screen Alerts table, printed p. 28

Checked

October 2026

We use the maker's document to read the code and do not reproduce it. Always confirm the complete model and control before applying these steps.