LG · Oven Moderate

F21 — Steam sensor open

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

Keep using it?
Yes, with caution
DIY difficulty
Moderate
Verified against
lg.com ↗

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.

Why it happens

Typical signs: 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

  • Open steam thermistor Sensor in the steam generator chamber failed open.
  • Disconnected sensor harness Connector backed off during heating/cooling cycles.
  • Corrosion at the sensor terminals Mineral scale or moisture corroded pins.
  • Main control board steam input failed Board cannot read the sensor circuit.

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.

  • LG oven steam sensor Common replacement
    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 — Rear Cooktop Sensor Error (Zone Varies by Model) F1 Range F1 — Upper Oven Thermistor Error

All LG codes → All oven codes →

Frequently asked questions

What does the F21 code mean on a LG oven?

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

What causes the F21 code on a LG oven?

Open steam thermistor: Sensor in the steam generator chamber failed open. Disconnected sensor harness: Connector backed off during heating/cooling cycles. Corrosion at the sensor terminals: Mineral scale or moisture corroded pins.

Is it safe to keep using a LG oven with a F21 error?

Yes, with caution. This is rated a moderate-severity fault.

How do I fix F21 on a LG oven?

Power-cycle the oven at the breaker for 60 seconds. If F21 returns, disconnect power and access the steam generator behind the cavity rear panel. Unplug the steam thermistor and measure resistance (~1090 Ω at room temp). Open = replace.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: lg.com.