Thermador Oven Not Heating — Diagnosis & Fix (Pro Harmony, Masterpiece, Steam & Conv Series)

Common Symptoms

• Oven displays E118, E121, or E122 error code on the panel
• Oven preheats very slowly — takes 30+ minutes to reach 350°F
• Oven reaches a plateau temperature (e.g., 250°F) but will not go higher
• Gas oven burner ignites on cooktop but oven burner will not light
• Electric oven bake element is visibly broken, bubbled, or has burn holes
• Temperature inside oven does not match the set temperature (verified by thermometer)
• Oven cycles on and off rapidly — short cycling
• Broil works but bake does not (or vice versa)

Most Likely Causes

1. 1

   Weak or Failed Igniter (Gas Ovens — Most Common)

   On Thermador gas ranges (PRDS366W, PRBS304, PRODS304), the oven igniter serves a dual purpose: it ignites the gas and, by drawing sufficient current (typically 3.2–3.6 amps), it opens the safety gas valve. A weak igniter that draws less than 3.2 amps will glow orange but never get hot enough to open the valve — gas never flows and the oven doesn't heat. Diagnosis: time the igniter glow. A healthy igniter opens the valve within 45–90 seconds. If the igniter glows for 2+ minutes without the burner lighting, it is weak and must be replaced — you cannot simply 'clean' a weak igniter. Thermador part WPW10179450 (or equivalent igniter for PRDS366W) costs $50–$90.

2. 2

   Burned-Out Bake Element (Electric Ovens — E121 Code)

   Electric Thermador ovens (PODS301, MED301, PODC302J) use a concealed or exposed bake element at the oven floor. When the element burns out — a visually visible crack, blistering, or burn-through — it breaks the circuit and the lower oven zone cannot heat. The control board detects the open circuit and logs E121. Test with a multimeter in Ω mode: a healthy Thermador bake element reads approximately 20–30Ω. OL (open circuit) = element failed, replace. The broil element is a separate component; if broil works and bake doesn't, the bake element is the first suspect.

3. 3

   Faulty Oven Temperature Sensor (E118 Code)

   The oven temperature sensor (also called the RTD probe or NTC thermistor) is a thin metal probe extending into the oven cavity — typically mounted at the upper rear wall. On Thermador ovens, the sensor should read approximately 1,080–1,090Ω at room temperature (75°F). A failed sensor reads OL, 0Ω, or drifts far from specification. The control board interprets the wrong resistance as an incorrect temperature and logs E118 — preventing the oven from completing a preheat. On PODS301 and MED301 models, the sensor is accessible from inside the oven cavity (two screws, pull the probe straight out). Thermador part WB21X22134 or equivalent costs $30–$60.

4. 4

   Burned-Out Broil Element (Electric Ovens — E122 Code)

   The broil element (upper heating element) failing triggers error code E122 on Thermador electric ovens. While the bake element handles standard cooking, the broil element also assists in preheat on some Thermador convection models. A failed broil element shows OL on a multimeter (healthy: 10–18Ω). E122 can also appear if the broil element has a partial break — the element appears intact but has developed a high-resistance spot. Visual inspection under bright light often reveals a hairline crack.

5. 5

   Failed Oven Control Board

   If the igniter or heating element and temperature sensor all test within specification, the oven control board (relay board) may have failed. On Thermador models, the relay board controls the voltage to heating elements; burned relay contacts or a failed triac can prevent the bake or broil circuit from energizing. Thermador control boards are expensive ($200–$450) and typically require professional installation to match the exact firmware revision. A control board replacement on a Thermador oven is generally worthwhile only if the unit is under 10 years old.

6. 6

   Gas Valve or Ignition Module Failure (Gas Ovens)

   If the igniter tests at full amperage (3.2–3.6A) but the gas still does not flow, the gas safety valve has failed mechanically — the bi-metal inside the valve did not open despite sufficient igniter heat. A failed gas valve is less common than a weak igniter but does occur on older Thermador ranges. The ignition spark module (visible spark ignition on some models) can also fail, preventing the spark electrode from arcing. Thermador gas valve replacement is an advanced repair requiring gas line disconnection — professional service is strongly recommended.

Quick DIY Checks

1. 1SAFETY FIRST — Gas oven: Turn off the gas supply valve (behind the range or at the main shutoff) before accessing any burner or igniter component. Unplug the range from the wall outlet. Electric oven: Switch off the dedicated 240V circuit breaker at the panel before accessing bake or broil elements. Thermador Pro Harmony ranges require a 240V/50A circuit. Do not reach inside the oven cavity while the unit is energized.
2. 2READ THE E-CODE — Press the CANCEL/OFF button on the Thermador control panel to clear active errors, then initiate a preheat cycle and observe what code reappears. Common codes: E118 = lower temperature sensor; E119 = upper temperature sensor; E121 = bake element open; E122 = broil element open; E128 = temperature runaway (sensor reading too high). On PODS301 and PRODS304 models, service mode is accessed by pressing and holding BAKE + BROIL for 5 seconds simultaneously — the display will show the fault log. Note the code before proceeding.
3. 3GAS OVEN — TIME THE IGNITER — Turn on the oven to BAKE at 350°F and observe the oven igniter glow through the oven window or lower oven drawer opening. Use a stopwatch. A healthy igniter glows bright white-orange and opens the gas valve within 45–90 seconds, at which point you hear and see the burner flame ignite. If the igniter glows for more than 2 minutes without the burner lighting, the igniter is weak and drawing insufficient current. The igniter is accessible by removing the oven bottom panel (typically 2 screws) — the igniter is mounted directly to the burner assembly. Replacement cost: Thermador PRDS366W igniter (WPW10179450 or equivalent) $50–$90.

Repair vs Replace

Recommended Tools & Parts

• Thermador Oven Igniter (Gas Models)

  Replacement flat-style oven igniter for Thermador Pro Harmony and Masterpiece gas ranges (PRDS366W, PRBS304, PRODS304). Restores heating when igniter glows but gas valve never opens. Draws 3.2–3.6A when healthy. Verify by full model number.

  $50–$90

  Buy on Amazon →

• Thermador Bake Element (Electric Ovens)

  Replacement bake element for Thermador electric wall ovens (PODS301, PODC302J, MED301). Fixes E121 error code and no-heat on bake. Healthy resistance: 20–30Ω. Match by full model number and element watt rating.

  $40–$75

  Buy on Amazon →

• Thermador Oven Temperature Sensor (RTD Probe)

  Replacement oven temperature sensor for Thermador wall ovens and ranges. Fixes E118/E119 error codes and temperature inaccuracy. Spec: approximately 1,080–1,090Ω at 75°F room temperature. BSH part 00487197.

  $30–$60

  Buy on Amazon →

• Thermador Broil Element (Electric Ovens)

  Replacement upper broil element for Thermador electric ovens. Fixes E122 code and no-broil. Healthy resistance: 10–18Ω. Model-specific — verify part number via BSH parts portal using full model and production date (FD) code.

  $45–$85

  Buy on Amazon →

• Digital Multimeter

  Required for testing igniter draw, element resistance, and temperature sensor ohms. Use DC current clamp or inline ammeter for igniter current test. Resistance mode for element and sensor testing.

  $20–$45

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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