Locate and pull out the turn signal socket for easy reach while testing. Be sure not to yank out any connections while doing so because circuits found in cars mostly use small wires that might break quite easily.
Connect a test light’s ground clip to a suitable electrical ground. The most convenient ground source to use is the negative terminal of your car battery. Test lights are very handy tools for testing wire connections, suitably designed with a sharp probe for piercing through wire insulation and a ground terminal to complete the circuit for the light inside the gadget to work.
Turn on either one of the turn signals so that current is running through the wires in the circuit.
Test the ground wire to see it is working properly. In most cases, the ground wire’s insulation is white or black in color for differentiation. Use the test light to determine the ground connection’s integrity by pinning the pointed end of the test probe to any point along the ground’s wire. The light should light up if the connection is good.
Test the signal wire leading to the right turn signal using the test light. This wire is usually plain green or green with a yellow stripe. Conduct this test in the same way you did while testing the ground.
Check the signal wire leading to the left turn signal, once more using the test light. This wire may be plain yellow in color, green with black strips, dark green, or gray with black strips. The bulb in the test light goes on when the circuit is OK and stays off when the circuit is faulty. Expect the same results when testing all the other wires in the circuit.
Replace the turn signal bulbs and check to see if this fixes the problem if all the tests turn positive.