Leaks & smells

Differential or gear oil leak

A leak from the differential (the housing at the center of the rear axle, or front axle on some vehicles) shows up as a thick, dark, strong-smelling oil, distinct from engine oil, usually pooling near a rear wheel or the center of the rear axle.

Difficulty
Moderate
Time
1–2 hours
DIY cost
$20–$60
Pro cost
$150–$500

Symptoms

  • Thick, dark oil puddle near a rear wheel or the center-rear underside of the car
  • Strong, distinct sulfur-like smell (gear oil has a different odor than engine oil)
  • Sometimes a whining or humming noise from the rear of the car while driving
  • Fluid seen on the inside of a rear wheel or near axle seals

Likely causes

  • CommonWorn axle sealThe seals where the axles exit the differential housing wear over time and let gear oil leak out, often visible on the inside of a rear wheel.
  • CommonLeaking differential cover gasketThe cover bolted to the back of the differential housing has a gasket that can harden and leak with age.
  • PossibleWorn pinion sealThe seal where the driveshaft connects to the differential can wear and leak, usually visible at the front of the differential housing.
  • Less commonOverfilled differential from a previous serviceToo much gear oil can be pushed out through a seal or vent that isn't designed to hold the excess.

What to do

  1. Identify the fluid as thick, dark, and distinctly strong-smelling (different from engine oil) to confirm it's gear oil rather than engine or transmission fluid.
  2. Check the differential cover for wetness or a visible drip line along its bottom edge.
  3. Check each rear wheel's inner side and the axle seal area for oil residue.
  4. Check the pinion seal area (where the driveshaft meets the differential) for leakage.
  5. Have the differential fluid level checked and topped off if low, and the specific leaking seal or gasket repaired to prevent gear damage from running low.

Tools and parts

  • Flashlight
Gear oil matching your vehicle's differential specificationSearch: “gear oil 75W-90”
Differential cover gasket or axle seal for your vehicleSearch: “differential gasket”

When to call a pro

See a mechanic if you hear a whining or humming noise from the rear of the car along with the leak, since running low on differential fluid can cause gear damage.

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.