I have a 2004 gmc sierra, left rear turn signal won't work since I replaced bulb 3157.

There should two separate bulbs for marker and blinker. It sounds like a bulb is burnt out, bad ground, melted socket or a wiring anomaly. Are you certain it's supposed to have 57 series dual element bulbs for blinker and markers?

i don't know. blinker slows to normal when both bulbs work. using volt meter or similar, test rear socket for a
continuity to ground. if OK test socket for 12 volts from
socket to ground. if OK, bulb is not making contact. if not
OK, note wire colour and look for damaged or loose
connection from front. check white back up light as well.

Check the socket that holds the bulb at the left rear. Make sure it's clean and wires are in it. Next make sure you have power and ground in socket. If you have all that try another bulb.

There may be a short in the front socket. Remove the bulb and inspect the socket for damage. If corroded, clean it up or replace it.

Just had a similar question last week, and the solution was a bad socket/bulb holder.
You might try and see if there is current in the brake light wire leading to the bulb holder.
I have used a jumper wire and a safety pin to test the wire behind the socket.
I pierced the plastic insulation of the brake light wire with the point of the safety pin, and either use a tester or even a good working bulb and a good ground wire.
Be blessed.

is there power at the light socket? if so try another bulb, should be a double filament bulb like a 3157

#3157

You 2004 Ford F150 use OEM 3157 bulbs.
Inside these bulbs,there are TOTAL of TWO bulb filaments.
When one of the two burned out,it will cause turn signal problem.

Stock up with 4 bulbs before you start.
Auto part store sell then is pair.
I recommend you replace then in part and save the good on for emergency.
I attached a photo of the 3157 bulb below.
As you can see there are two filaments inside the bulb.
Check the bulb one by one and you will get the signal system working.
==== There are protective grease inside the base of the socket.
Make sure it's not blocking the four contact points (both side of the socket). ====

Have you checked the rear turn signal on the same side? Usually when one (1) turn signal stops blinking another bulb on the same side has failed. It can be rear or side lights but most of the time it is the rear light on the same side that has failed so the front light will not blink.

Also I would check to see if there was 12V when the turn signal is selected and a ground on the bulb socket. (signal off)

Does the bulb have one or 2 contacts on the base of the bulb besides the barell? If it is a 2 contact bulb (1157) then there are separate circuits for the turn and parkling lights. Follow the socket wires back to the connector and check to see if there is corrosion in the socket or the pigtail connector. All of the terminals socket and pigtail connector should be clean. Also check for signs of any breakdown in the wires especially around terminal ends.

Your dealing with a 2 wire series circuit... power and ground. It takes both to make the bulb light.

Check the ground for Zero (0) Ohms and power for 12V. One of them has to be missing and you will need to find out where the break is in the circuit.

Often the ground is provided by the turn signal selector but if the selector is bad all lights on the same side would not work. If the rear light works and the front light does not then your problem is probably very near the front light.

1. Dirty socket terminal
2. Poor pigtail connector connections
3. Broken chassis ground or power wire

Thanks for using FixYa.

Kelly

If the left turn signal blinks fast and the rear one does not light up (working in a dealership i see this All the time) almost always it is dew to one of the other bulbs that is burned out. in your case the LF one works fast and the LR one is out. You say you already replaced the bulb- but i would recheck that area again-make sure the bulb you installed was the correct one- it should most likely be a 3157 or 1157 bulb- but i dont know off hand it may be a 3156 or a 1156 bulb. If it is the right bulb make sure it is not blown itself.- make sure you did not accidently install the bulb backwards. finally check the socket- make sure it is free from corrosion and make sure the metal is making contact with the bulb. I hope this works. check if there is power ond ground going to the socket if not with a test light to make sure you dont have a broken wire. This failure will not be caused by a blown fuse.