Starting problems

Battery warning light on and car won't start

A battery light that's on before a no-start usually means the charging system (alternator) wasn't keeping the battery charged, so the battery ran down even though it may be otherwise healthy.

Difficulty
Moderate
Time
30–60 minutes
DIY cost
$50–$400
Pro cost
$300–$700

Symptoms

  • Battery/charging warning light was on before the car died
  • Dimming headlights or interior lights while driving beforehand
  • Car eventually wouldn't crank or cranked slowly
  • May have had electrical issues like slow power windows recently

Likely causes

  • CommonFailing alternator not charging the batteryIf the alternator stops charging, the battery slowly drains until the car won't start.
  • CommonWorn or slipping serpentine belt (drives the alternator)A loose or worn belt can cause the alternator to spin inconsistently or not at all.
  • PossibleBad alternator connections or wiringA loose plug or corroded wire can interrupt charging even with a good alternator.
  • PossibleBattery that's simply worn out and can no longer hold a chargeAn old battery can show similar symptoms even with a healthy alternator.
  • Less commonBlown fuse in the charging circuitSome vehicles fuse the alternator output, and a blown fuse stops charging entirely.

What to do

  1. Try jump-starting the car; if it starts but the battery light stays on, the alternator likely isn't charging it.
  2. With the engine running (if it starts), check whether the battery light goes off; if not, don't turn the engine off, and get it looked at soon.
  3. Check the serpentine belt for visible cracking, glazing, or looseness, and listen for a squeal that suggests slipping.
  4. Have the alternator output tested with a multimeter or at an auto parts store if you can get the car running.
  5. If it won't start even with a jump, treat it as a charging system failure and have it towed rather than repeatedly jumping it.
  6. Avoid driving on a battery light for long, since you can strand yourself once the battery fully depletes.
  7. Replace the battery only after confirming the alternator is charging properly, or the new battery will drain too.
  8. Check charging system wiring connections for corrosion or looseness if the alternator itself tests fine.

Tools and parts

  • Jumper cables or jump box
  • Multimeter
AlternatorSearch: “alternator for [year make model]”
Serpentine beltSearch: “serpentine belt for [year make model]”

When to call a pro

A shop can quickly load-test the alternator and battery together to tell you which part (or both) needs replacing.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.