What else to do/clean when jets clog in furnace?

my boss said his heat was broken and I work from his house on Wednesdays. I asked him to show it to me. He said it needs a flame sensor bc the flame shuts down immediately. I thought maybe it was an airflow issue and removed the cover to see if I take air was a problem. Nope. But I did notice the flame sensor looks like it’s on the last jet in the gas line, AND that jet wasn’t firing. I rubbed my finger over the jet and got it firing in just a few minutes. And this is after TWO hvac guys had been at his house and didn’t solve the problem. Apparently the first guy did do a cleaning, but it looks to me like the jets might be a little rusty. I’m going to guess maybe it was a rust flake.

Should my boss replace the entire pipe that’s beyond the gas metering device thing? He says he’s already gotten that replaced and has a spare (old?) one in a box.

What else to clean when furnace gas jets clog?
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