Bosch Vision 500 NG Dryer - Burner Won't Ignite

[KenB]

Have a BOSCH Vision 500 Natural Gas Dryer.  Dryer would not heat and could smell gas after trying to start.  Dryer is approx 4-5 years old, but looks like new inside.  Dryer tumbles fine and air dries, but burner will not ignite. Belt sensor is OK.  Vent is clean and all lint has been removed from dryer and vent pipe.  Wiring connectors are all fully seated and clean.

Before I had time to take the dryer apart and attempt to diagnose the problem, I simply ordered a new igniter in hopes that that was the issue.  Replaced igniter (Bosch #00491648) last night, but that was not the problem.  Igniter glows red hot for perhaps 15-20 seconds, and then shuts off.  Does not appear that gas burner even attempts to ignite.  After I shut off dryer, I can smell gas.  According to literature, it appears the Flame Sensor or the Gas Valve Coils may be at fault.  Any thoughts on which one is at fault or how to check operation  so I know what to order.  Thank you in advance for your advice.

[sh2sh2]

Sounds like the coils

[darren412]

when the glow bar is at its peak of glowing try tapping the side of the gas valve at the coils also . If the burner lights it’s possible you may have a stuck gas valve or bad gas valve.  Meter check the other components first and make sure their good before coming to the conclusion that the gas valve is bad. Ohm out the flame sensor . You should read about 1-2 ohms .  If your reading infinity . Replace the sensor.  The flame sensor rarely goes bad.  It’s most Likely the coils if the flame sensor checks good and the glow bar has  already been changed with a new one and it’s checked for being good. You should be getting 120volts at the glow bar .  You also need to make sure there is not a break in the wiring harness wires for the ignition components. There is a way to test the coils with a meter but you need to have the right type of meter  for that.  Usually a weak set of coils will give off a small scent of gas sometimes.   A weak or bad coil will look fractured a sort of cracked look all around the coil. It will also have a swollen look at the gas valve end of the coil.  Good luck

[MoTeD]

Agreed. Sounds like the coils.