GUIDE AS 9G9638 - Caterpillar

Unburned gas in the air inlet and exhaust system may ignite when the engine is started. Personal injury and/or property damage may result.Before starting an engine that may contain unburned gas, purge the unburned gas from the air inlet and exhaust system. Refer to the topic on purging unburned gas in the "Starting the Engine" section.
The following events may cause unburned gas to remain in the air inlet and in the exhaust manifold:
Emergency stop
Engine overspeed
Unexpected engine shutdown
Unsuccessful attempt by the Electronic Control Module (ECM) to close the gas shutoff valve
Unsuccessful attempts to start the engineUnburned gas may remain in the air inlet and exhaust system after several unsuccessful attempts to start the engine. The unburned gas may increase to a concentration that may ignite during a successive attempt to start the engine.Perform the following procedure in order to purge the unburned gas:
If the Caterpillar Electronic Technician (ET) is not connected to the engine, connect Cat ET to the engine.
Verify that the value of the "Engine Purge Cycle Time" parameter is equal to 10 seconds less than the value of the "Cycle Crank Time" parameter.
Set the engine control to the START mode. The engine will crank for the "Engine Purge Cycle Time". Then, the gas shutoff valve will be energized and the ignition will be enabled. The engine will start.
Continue with your previous procedure.