The older your Sentra is, the more likely it is that your ignition system can have problems. However, older models, such as late 90s and earlier, are also easier to troubleshoot than newer models. Whatever the age, troubleshooting the ignition system involves testing the spark plugs and their wires along with the ignition coils.
Disconnect a spark plug wire from its plug. Make sure the Sentra's engine is off when you do this.
Attach a spark tester to the wire and ground the tester to a metal spot on the engine. If a tester isn't available, wrap a thick, dry cloth around the wire boot and hold it a quarter inch from the electrical ground.
Crank the engine with a remote starter or the help of another person. Look for a strong blue spark to appear on the tester or wire. A yellow or orange spark is weak.
Reconnect the spark plug wire and repeat the test for all the others. If a weak spark or no spark appears at any individual wire, the wire itself likely needs replacing. If there's trouble at all wires, the problem may be the distributor or ignition coil.
Measure the primary resistance on the ignition coil. Remove the coil's primary, ground and high tension wires with the ignition off. Use an ohmmeter on the primary and ground terminals, then the ground and high tension ones. Replace a coil whose resistance fails specification.
