Oven heats but does not reach the set temperature, food takes much longer to cook than expected.

Safety first: Disconnect power before accessing internal components.

• Verify with thermometer: Place an oven-safe thermometer in the center of the cavity and compare actual temperature to the set temperature over 20–30 minutes.
• Check sensor: Test the NTC sensor resistance at room temperature and at a known elevated temperature (e.g., 150°C if you have data). Replace the oven NTC sensor if readings are out of specification.
• Inspect elements: Look for cold spots or uneven glow on electric elements. A partially failed element may still heat but not fully. Replace any suspect heating element.
• Calibrate (if available): Some Tristar models allow temperature offset adjustment in the settings menu. Apply a small offset (+/− 10–20°C) if the oven is consistently off by a similar amount.

If underheating persists after sensor and element checks, the control PCB's temperature regulation may be faulty and require replacement.