My car is showing an electrical problem (I suspect it is electrical). As an intermediate DIY what should be the first things I check, and how easy is it to check them?

I'm thinking of Fuses, Relays, battery, alternator, etc.

I understand that cars have fuse boxes, I also vaguely understand that a fuse can be blown and that I can visually inspect it.

I understand that cars have relays but I don't understand what they do and how to know if there is one bad one that is causing the problem. and how to fix them.

Should I disconnect the battery when checking for this? How to disconnect the battery?

How to test if the battery is working fine? and an alternator?

I know this is a general question.. But I strongly feel these types of questions are extremely useful for novices like me. Would be great if i could understand what are the most common types of electrical failures and what to check first.

  • 2
    This isn't the right way to diagnose any type of problem. You start from the symptoms and narrow down the cause, then make relevant checks. That is often common sense - if one light bulb has gone out and everything else works normally, the most likely reason is that the bulb has blown, not that your battery and alternator are faulty! Checking things at random is usually a waste of time, and replacing things that you think might be causing a problem is usually a waste of money as well.
    – alephzero
    Jul 19, 2019 at 10:59
  • @alephzero I suggest turning this comment as an answer
    – gota
    Jul 19, 2019 at 11:01
  • This is too broad @gota, there is no good answer for it.
    – GdD
    Jul 19, 2019 at 11:11
  • Remember the first rule of electrical faults : whichever end you start, the fault will be at the other...
    – Solar Mike
    Jul 19, 2019 at 11:25
  • 1
    You should break this down into multiple questions. e.g. How to test a battery. How to test an alternator. How to test a relay. How to test ..... You will then get good answers that will be useful.
    – HandyHowie
    Jul 19, 2019 at 13:15

1 Answer 1


As @alephzero said in the comments, this very much depends on the problem - so the first thing to do is to identify the symptoms you're experiencing. Look at the problem you've got, and work out what part, or parts of the electrical system are affected.

If it's more than one thing, then the key question is "what do these things have in common?" - for example, if all the lights in your left-rear light cluster aren't working, then the common element is that cluster, so you'd want to check the earth connections there. If it was both your headlamps, then you might want to look at the switch or relays controlling them.

There are separate questions on here about checking batteries and alternators, but you'd only want to do so if the symptoms suggested they might be at fault.

Learn how to use a multimeter - it's your secret weapon when it comes to anything electrical. There's questions on here about that too.

Multimeter - Basic Functionality and Howto

How do you test analogue electrical circuits?

How do I test my car battery?

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