Oldminiman has covered most things, however, it would be prudent to check the spark plug gaps and the HT leads. Incorrect spark plug gaps and defective leads could lead to a slight misfire, the knock sensor on the engine could possibly detect this and throw up a fault in the system's monitoring.
Plug leads are usually pulled off by the lead by many people, this strains the conductor and can break them or pull the connector from the conductor, check the obvious, are the plug leads firmly attached to the plugs.

The knock sensor basically listens to the engine, the frequency it operates at may be outside its normal operational range.

What you did sounds fine but I would check around the engine compartment to see if there are any disconnected sensor leads. It is quite possible that one was knocked or pertially disconnected and has now fallen off sending a warning to the EMU, hence the warning light. I can't really see that anything you did to service the car would cause this type of failure, not even loose plugs which would just cause compression loss past the sealing rings. Check also the LT connector to the coil although this would not normally cause the warning light to come on but it would cause the engine to misfire and then fail to start if it came off.

Have a really good look round and check that all sensors visible are connected.

GOOD LUCK

OLDMINIMAN

This is a general comment, as I have no experience with the vehicle you mention:

It sounds like it may not be firing on all cylinders - are the ignition leads in good condition, and correctly connected (and not mixed up)?

Are the plug gaps correctly set to what the manufacturer recommends?
Check the plugs are not damaged - have the insulators been damaged (cracked), by say dropping?
Are the plugs in tight enough?

Just some ideas.....

1985 Veg oil burning Fourtrak "Rocky".