Uneven cooking in a gas oven is a frustrating but common problem, and the good news is there are several straightforward causes to check before calling anyone out. In most cases, the culprit is a partially blocked burner, a faulty igniter, poor rack positioning, or a thermostat that's drifted out of calibration. Some fixes are simple DIY jobs; others involve gas components and must be handled by a licensed professional.
How a Gas Oven Heats — and Why It Can Go Wrong
Unlike an electric oven with separate elements, a gas oven typically uses a single burner at the base (or sometimes the rear) of the oven cavity. Heat rises and circulates naturally, or is assisted by a fan in fan-forced models. Because the heat source is localised, any disruption to the flame or airflow can create hot and cold spots — often noticeably worse on one side.
Common Causes of One-Sided Uneven Cooking
1. Partially Blocked or Dirty Burner Ports
The oven burner has a row of small ports (holes) through which gas flows and ignites. If food debris, grease, or cleaning product residue blocks some of these ports — particularly on one side — that section of the burner won't produce a full flame. The result is a cooler zone on that side of the oven.
What you can do: With the oven completely off and cooled, remove the oven floor panel (consult your appliance manual) to access the burner. Use a soft brush or a toothpick to gently clear visible debris from the ports. Never use water directly on the burner or poke metal objects into gas orifices.
2. Weak or Uneven Igniter
The igniter doesn't just spark the gas — in many gas ovens it also acts as a safety valve sensor. A weakening igniter can cause the burner to light unevenly or only partially, leading to a lopsided flame pattern and uneven heat.
Important: Replacing an igniter involves disconnecting a gas appliance component. In New Zealand, any work on the gas components of an appliance — including internal parts connected to the gas train — is restricted work under the Gas (Safety and Measurement) Regulations and related legislation and must be carried out by a licensed gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board. Do not attempt this yourself.
3. Thermostat Calibration Drift
Over time, oven thermostats can drift, meaning the temperature inside the oven doesn't match the dial setting. If the thermostat sensor is positioned to one side of the cavity, a calibration issue can make one side run hotter or cooler than the other.
What you can do: Buy an inexpensive oven thermometer from Bunnings or Mitre 10 and place it in different positions inside the oven to map the temperature variation. If you find a consistent difference of more than 15–20°C between sides, the thermostat likely needs professional attention.
4. Poor Airflow or Oven Rack Position
Gas ovens rely on convection — hot air rising from the burner. Overcrowding the oven, using oversized trays that block airflow, or placing food too close to one wall can all cause uneven results.
What you can do: Use baking trays that are at least 25–30 mm smaller than the oven cavity on each side. Rotate dishes halfway through cooking. If your oven has a fan-forced setting, use it — it significantly improves heat distribution.
5. Oven Not Level
If your oven or range isn't sitting level, heat and moisture can pool to one side, affecting how food cooks — especially baked goods.
What you can do: Place a spirit level on the oven shelf. Most freestanding ranges have adjustable feet; check your manual and adjust as needed.
6. Faulty or Damaged Door Seal
A worn or damaged door gasket lets heat escape unevenly, often more from one side if the seal is only partially compromised. Run your hand carefully around the door edge (with the oven on) to feel for escaping heat.
What you can do: Door seals on most ovens are a user-replaceable part — check your appliance brand's website or contact the manufacturer for the correct replacement part.
A Quick Diagnostic Checklist
| Check | DIY Friendly? | Notes |
|---|---|---|
| Clean burner ports | Yes | Oven off and cold |
| Oven thermometer test | Yes | Cheap and informative |
| Level the oven | Yes | Adjust feet carefully |
| Check door seal | Yes | Replace if cracked or loose |
| Igniter replacement | No — restricted work | Licensed gasfitter required |
| Thermostat replacement | No — restricted work | Licensed gasfitter required |
| Gas valve or regulator issues | No — restricted work | Licensed gasfitter required |
When to Call a Professional
If your checks point to a faulty igniter, thermostat, gas valve, or any other internal gas component, stop there. Under New Zealand gas safety legislation, work on gas appliance components that form part of the gas train is restricted work in New Zealand and must only be carried out by a gasfitter licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB). Attempting gas repairs yourself is illegal and potentially life-threatening.
If you ever smell gas near your oven, turn off the appliance at the isolation valve, ventilate the area, and call a licensed gasfitter immediately. Do not use electrical switches or open flames. For guidance on gas safety, refer to WorkSafe NZ.
A qualified gasfitter can also certify that your appliance is operating safely and efficiently — important for both your safety and your energy bills. You can find licensed gasfitters in your area at tradedirectory.co.nz.
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