Code 42 in 1989 Chevy vehicles is related to the electronic spark timing (EST) circuit that the on-board computer uses to control ignition timing advance according to engine load. This circuit is part of the ignition module in high energy ignition (HEI) and distributor-less ignition systems. The most common cause for a code 42 is a faulty ignition module, and replacement of the ignition module is a fairly straightforward procedure that can be accomplished in about an hour.
Remove the distributor cap with a screwdriver by turning the four locking tabs on the sides of the cap counterclockwise 1/4 turn. Lift the cap off the distributor. Remove the two Phillips head screws that secure the ignition rotor to the top of the distributor shaft located under the cap then lift the rotor out of the distributor.
Remove the two electrical connectors from the black rectangular module bolted to the base of the distributor. Use a flat head screwdriver to gently pry the connectors off the ends of the module. Remove the two 1/4-inch screws that attach the module to the base of the distributor then slide the module out of the distributor.
Coat the back of the module with a light film of diaelectric grease. Slip the new module into place on the distributor and install the retaining screws. Plug the electrical connectors onto the new module.
Reinstall the ignition rotor and distributor cap. Start the engine and test-drive to verify the engine runs properly.
Locate the coil pack by following a spark plug wire from the spark plug to the coil pack. Unplug the two electrical connectors from the end of the module by unscrewing the 8 mm retaining bolt with an 8 mm socket and ratchet while pulling the connector from the module. Remove the coil pack and module by unscrewing the six 5.5 mm screws that attach it to the ignition bracket with a socket and ratchet.
Pull up on the old coil pack to remove it from the old module. Install the old coil pack onto the new ignition module. Install the assembly onto the ignition bracket and tighten the 5.5 mm screws securely.
Plug the electrical connector into the module and tighten the 8 mm retaining bolt. Start the engine. Test-drive the car to verify that the engine accelerates properly.
