Back LEFT XLO simmer burner doesn't ignite. It has always had a spark. When air is blown into the gas tube on the left side, air exits out front burner ( the other 3 burners work perfectly now that we just installed 2 new potentiometers ( but not a simmer controller - blue plastic box) but no air exits from the back burner. Suspect something is blocked or not opening to let gas into left burner. Where is the most likely blockage? What part should we replace? We have NOT taken the whole top off of the cooktop but realize we may have to. Want to know what we might need so we can purchase a variety of parts and do this once. We live in Mexico so hence need to order parts from the US before we start the complete "take apart". Local Thermador dealer and service department are useless. Hope someone can help us. Stove is from the US, purchased 1997-1999. We were not given any manuals when we purchased the house from the builder. We only have the wiring diagram but no diagram of gas flow or other parts.
Thanks.