My lpg fryer IFS40 burns yellow flame and on the recent inspection tech supposedly found corroded heat exchanger. The fryer is only half a year old and still under warranty but the imperial only offer

yellow flame and black smoke is incorrect air /gas ratio
there is an air shroud over the jet that controls the air flow mix and it can be adjusted forward or back to open / close the air inlets holes
could be that the jets are incorrect for LPG as the indications are the same if running on LPG with natural gas jets

Talk to the manufacturer or check your owners manual. The jets in the burner must be slightly smaller for butane. The flame will burn yellow with black soot/smoke if you don't change them.

Use a #74 as a start. The #63 is actually for natural gas.

The burner is blocked need cleaning/ the air ports to the burner are blocked lack of air causing yellow flame and soot

Needs a service asap.

It sounds like your gas line either is clogged or your heating tubes are clogged up with oil/dirt/gunk? You can blow these out with a small hand held compressor or if you have a small enough brush, you can use that to clean it. The compressor is the quickest way, but have you cleaned this unit at all? Using a fryer boil out and running the fryer will also help clean out your holes, but again, the compressor works best.

check the burner orifice and gas valve spring kit if it is suited for propane use.remove carbon build up inside the fire tubes.check also if the gas pressure is correct.

Make sure the regulator assembly on the gas valve is for propane. Sounds like the gas pressure is too low and the flames aren't going into the burner tubes. Make sure they didn't install an appliance regulator on the back of the fryer. This is not needed as the fryer has a regulator on the gas valve. You can unscrew the cap on the regulator assembly and turn the plastic screw inside all the way in. DO not turn with alot of force. Easy does it.
Tom

Without buying new burners, try adjusting the air so just the very tip of the flame starts to turn yellow or white.

I do not know where you are located but if you have a restaurant supply or a repair tech near you they can get you a replacement kit for you

If the unit starts off fine, and then loses some of it's heat, I would suspect that your regulator is freezing up. This can be caused be a bad tank or regulator. You'll have to swap it out to test it.

If the gas supply is good (reg and tank), you need to inspect the heat exchange tubes. Look where the flame shoots through the fryer. Use a flashlight. Look for LARGE amounts of black soot. If it is sooty, blow it out with compressed air. Once you get it cleaned out, you need to add more air to your mix. There should be a disk at the base of the burner.

If you have access to a manometer and orifice drills, you could adjust the flame to it's maximum. But as I said, I expect you have a regulator problem.