Main oven does not heat correctly, display shows F3 during preheat or shortly after starting.

Safety first: Isolate the cooker from the mains supply before removing any panels.

Steps to check:

• Access sensor wiring: Remove the rear panel. Locate the sensor harness from the oven cavity to the control PCB.
• Inspect for damage: Look for crushed or melted insulation where the harness passes near the oven casing or elements. Separate any wires that are touching bare metal and wrap with high-temperature insulation tape or replace the harness.
• Dry connectors: If there are signs of moisture or steam ingress at the connector, unplug it, dry thoroughly, and clean with electrical contact cleaner.
• Measure resistance at PCB end: With the sensor connected, measure resistance from the PCB connector pins. Compare to the reading directly at the sensor. If the reading at the PCB is much lower, the harness is shorted and should be replaced.
• If wiring and sensor are OK: The control PCB sensor input may be defective. Replacement of the main oven control PCB is recommended via an authorised Rangemaster service agent.