The procedure to clear the codes is:
turn on key
wait 3 seconds
depress pedal 5 times (approx. 1 sec intervals)
wait seven seconds
hold down pedal until hour meter alternates between hour and code
lift pedal
hold down pedal until only hour flahes

A stop watch is very helpful, as the 3 and 7 second parts are critical,this works better with the engine warm.

By the way I would check the converter with a pressure guage, remove the 17mm plug with a phillips screw that is on top of the converter, pressure should be between 4-5 psi, also open the drain on the bottom of the converter, will have to remove battery for access, there is a 12mm plug. There is also a filter in the lock-off. If the pressure is correct you have eliminated the lp tank, converter , and lock-off as being the problem. If there is no pressure check the lpg relay, just some helpful tips. The fuel pump relay is not used on lp trucks so you have an extra relay in the blue box.