F21 Moderate Moderate DIY

LG Oven F21 — Steam sensor open

LG Oven · Error code F21

What F21 Means

On steam-equipped LG ovens, the steam temperature sensor (thermistor) circuit reads open. Steam-assisted cooking and steam clean are disabled.

Symptoms

  • F21 or F-21 on display
  • Steam Cook / Steam Clean cycles will not start
  • Standard bake still works
  • Cycle cancels shortly after pressing Start on a steam mode

Common Causes

1
Open steam thermistor

Sensor in the steam generator chamber failed open.

2
Disconnected sensor harness

Connector backed off during heating/cooling cycles.

3
Corrosion at the sensor terminals

Mineral scale or moisture corroded pins.

4
Main control board steam input failed

Board cannot read the sensor circuit.

How to Fix It

  1. Power-cycle the oven at the breaker for 60 seconds.
  2. If F21 returns, disconnect power and access the steam generator behind the cavity rear panel.
  3. Unplug the steam thermistor and measure resistance (~1090 Ω at room temp). Open = replace.
  4. Inspect the harness for crushed insulation or corrosion at the connector.
  5. Reseat the connector at the main board side as well.
  6. If sensor and wiring are good, replace the main control board.

Parts You May Need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

Common replacement LG oven steam sensor View on Amazon→

More LG Oven Codes

5b 5b — Sabbath Mode active (not a fault) CF1 CF1 — Left-front cooktop sensor open/short CF2 CF2 — Left-rear cooktop sensor open/short CF3 CF3 — Right-front cooktop sensor open/short CF4 CF4 — Right-rear cooktop sensor open/short F1 Range F1 — Upper Oven Thermistor Error

All LG codes → All oven codes →

Verified against manufacturer documentation: lgparts.com

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.