Low power, lights pulsing

Hi and thanks for using 2CarPros.

The light issue sounds like a problem with the alternator voltage regulator or a bad ground. Keep in mind, the RPM can be related to a power issue as well. With that, I need you to check a few things. First, I want to conform that the battery terminals are clean and tight.

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

IF they are good, then check the alternator. It's easy to do. All you need is a volt meter or a multi-meter.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/how-to-use-a-voltmeter

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Also, load test the battery.

https://www.2carpros.com/articles/car-battery-load-test

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Although idle speed is an issue, since the lights are affected, I have a feeling they are related. If it was only idle speed, then I would be questioning other things such as an engine vacuum leak, a faulty idle air control valve, fuel pressure, and the list goes on.

With that in mind, start with the electrical tests I suggested and let me know what you find. Also, make sure the battery terminals are clean and tight at the battery and where they attach.

Let me know what you find.

Joe

I’ve repaired dozens and dozens of these Focus’ for battery cable issues, which sounds like you are having. While it’s running with headlights on bright, A/C on high, defrost on, radio on, check to feel how warm the battery cables feel near the battery. If they’re super hot you’ll need to replace them and I even add an extra ground with 4 gauge wire. And do not use the cheap bare wire clamps unless you solder or crimp on lugs to the cables so that they will bolt on securely.