The firing order is the sequence in which the cylinders fire in the engine. Different engines have different firing orders, so it's important to consult the vehicle's repair manual to find the correct firing order for your engine.
If the spark plug wires are not installed in the correct order, the engine will not run properly. The engine may misfire, stall, or even backfire.
So to ensure that the engine is running properly, make sure to install the spark plug wires in the correct order.