Bosch Gas Burner Won't Light: Causes & Fix

A Bosch gas burner that won't light has broken somewhere in a short chain of events: the spark fires, gas flows, the flame sensor confirms heat, and the valve stays open. Most failures are at the very start — a wet or dirty igniter that sparks weakly — and most "lights then dies" failures are at the end, where the thermocouple isn't holding the valve. Bosch's FlameSelect burners, with their fine flame control, are less tolerant of the carbon that causes both.

The Bosch Ignition Chain#

When you turn a Bosch burner knob, six things happen in order: the knob mechanically opens the gas valve, the spark module fires every igniter at once, each electrode throws a spark across a 3-4mm gap to the grounded burner, the gas lights from that spark, the flame sensor detects heat within about 4 seconds, and then the spark stops while the thermocouple's own voltage holds the safety valve open. If the flame sensor sees no heat within those 4 seconds, the safety solenoid shuts the gas off. A failure at any one of those steps stops the burner from lighting or from staying lit — so the trick is figuring out which step broke.

The Causes, By Frequency#

1. Wet or Dirty Igniter (30% of cases)#

The ceramic igniter electrode sits in the burner well, exposed to spills, steam, and cleaning liquids, and when moisture bridges the gap to ground the spark shorts instead of jumping — you hear clicking but see no spark at the tip. Carbon from cooking builds up on the electrode and the facing burner body too, raising the gap resistance until the spark can't jump. Dry the igniter thoroughly (give it a day after cleaning, or use a hair dryer on low), and clean the electrode tip and the burner body with a dry toothbrush; for stubborn carbon, use fine 400-grit sandpaper on the metal tip only, never on the ceramic. This is the easiest and most common fix.

2. Burner Cap Misalignment (25% of cases)#

Bosch burners use removable caps that must sit perfectly flat and centered, and even a 2mm offset blocks the flame ports on one side so the gas exits unevenly and misses the electrode — on FlameSelect models with their fine 0.8mm ports, alignment matters even more. Caps get bumped off-center by cleaning or large pots. Lift the cap, clear any debris from the burner head's seating surface, and drop it back into its locating recess; Bosch caps have a tab or flat that matches a notch on the head, so the cap only seats one way.

3. Spark Module Failure (20% of cases)#

The spark module generates the high-voltage pulses for all burners. No clicking from any burner means a dead module or no power to it; clicking with a weak or absent spark on one burner means that channel failed internally; and continuous clicking that never stops points at the module or the flame sensor. Before condemning the module, confirm 120V is reaching it at its input connector, because a blown kitchen-circuit fuse can kill ignition power while the knobs still open the gas valves mechanically. The module lives beneath the cooktop, reached by removing the cooktop from the counter with its Torx T20 bracket screws.

4. Flame Sensor / Thermocouple (15% of cases)#

If the burner lights but goes out within several seconds of releasing the knob, the flame sensor isn't generating enough voltage to hold the gas valve. The thermocouple makes a small voltage when heated by the flame — on the order of 20-30mV — and that voltage keeps the safety solenoid energized; once it falls below roughly 15mV, the valve closes the moment the spark timer expires. Test by lighting the burner and holding the knob in the ignite position for about 30 seconds, then releasing: if the flame dies within a few seconds, the thermocouple is weak, and you can measure 20-30mV DC at its connection when the burner is lit. FlameSelect thermocouples are more sensitive because they distinguish many flame levels, so keep the tip clean with 400-grit sandpaper.

5. Gas Valve Solenoid (10% of cases)#

Each burner has a safety solenoid in its gas valve, controlled by the thermocouple, and if that coil burns out no gas flows even with the knob turned. Turn the knob to ignite and hold a hand over the burner — you should feel a little gas within a second, and none at all (with the supply confirmed open) means the solenoid isn't opening. On Bosch the solenoid is integral to the valve body, so the whole valve is replaced, which means disconnecting the gas line and leak-testing afterward — gas work best left to a professional.

Troubleshooting Order#

1. Check all burners. None click means power to the spark module (fuse, outlet); some click means a partial module failure or one bad igniter.
2. Watch for the spark in dim light — no spark means a wet, dirty, or cracked electrode, or a module fault.
3. Reseat the cap so it drops flat into its recess.
4. Test the hold — light it, hold the knob 30 seconds, release; going out means the thermocouple.
5. Feel for gas during an ignition attempt; no gas means the valve solenoid or the supply.
6. Try a manual light with a long match on low — if it lights and holds, the spark system failed but the gas path is fine.

DIY Difficulty by Component#

When to Call a Professional#

Cleaning and drying an igniter, reseating a cap, and sanding a thermocouple tip are all easy DIY jobs, and they resolve the large majority of no-light complaints. Where it gets serious is anything involving the gas valve: a burned-out safety solenoid means replacing the whole valve body, which requires disconnecting the gas line and leak-testing every connection afterward — gas work that belongs to a qualified technician. Spark-module replacement is more approachable but still means pulling the cooktop from the counter to reach it, so weigh whether you are comfortable there. And if you ever smell gas with the burners off, stop, shut the gas supply, and call the gas company before doing anything else.

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