The basic definition of a transmission is the part of a vehicle that transfers the power from the engine to the wheels. How a transmission works depends on whether or not the vehicle is an automatic or a manual.

Manual vs. automatic transmissions

A manual transmission has a set of gears that are lined up on a shaft. As the gearshift and clutch, located inside the car, are manipulated by the driver, the gears move into place. As the clutch is released, power from the engine is transmitted to the wheels. The amount of power, or torque, is dependent upon the gear that was chosen.

With an automatic transmission, the gears are lined up on the shaft, but the gears change by manipulation of the gas pedal inside the car. As the driver presses on the gas pedal, the gears shift up automatically depending on the current speed. If the pressure on the gas pedal is released, the gears cycle down, again depending on the current speed.

Transmission fluid lubricates the gears and makes it easier for them to move as the process of switching gears is completed.

How often do I need to replace the transmission fluid?

Again, this depends on if the vehicle is automatic or manual. In an automatic transmission, there is higher heat produced, which means that there’s going to be more carbon produced, which will contaminate the transmission fluid. Over time, these contaminants will cause the fluid to thicken and no longer do its job effectively. Manufacturer specifications for automatic transmission fluid vary considerably, from 30,000 miles to never. Even if the user’s manual says that the fluid will last the lifetime of the car, the fluid level should be checked periodically, in case a leak develops.

In a manual transmission, the recommendations can also vary widely, but for different reasons. Most manufacturers suggest 30,000 to 60,000 miles as the point at which you should change the transmission fluid in a manual transmission. However, vehicles with transmissions that are under ‘high stress’ should change the transmission fluid every 15,000 miles. ‘High stress’ for a manual transmission would be situations such as being driven on lots of short trips where gears are changed more often. If you live in a city and rarely put any highway miles on your vehicle, the transmission is under high stress. Other situations include lots of mountain driving, and any period when a new driver is learning how to use a manual transmission.

Signs you should check your transmission

Even if you haven’t reached the mileage threshold indicated by the vehicle’s user’s manual, the transmission should be checked if the following symptoms are noticed:

  • If a grinding sound is heard from underneath the vehicle while the engine is running, but the car is not moving.

  • If you have problems shifting gears.

  • If the vehicle slips out of gear, or if the vehicle doesn’t move when the gas pedal is pushed.

Occasionally, the transmission fluid can become contaminated to such a level that it needs to be flushed before the manufacturer specifies.

No matter the type of transmission, changing the transmission fluid is not a quick process that can be taken care of with a wrench and a nozzle. The vehicle will need to be supported, and the old fluid will need to be drained, and disposed of properly. Furthermore, the transmission fluid filter and gaskets will need to be inspected. This is the type of vehicle maintenance that needs to be left to licensed mechanics rather than attempted at home.

This article originally appeared on as How Often Do I Need to Replace the Transmission Fluid? and was authored by Keisha Page.

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