There are many fluids that run throughout your vehicle, but one of the most important to keep track of is the transmission fluid. Whether or not you should change it is not a matter of debate: Yes, you should. But how often this service should be performed varies by manufacturer and vehicle, and it’s open to debate.

The manufacturer’s maintenance schedule for many automatic transmissions doesn’t call for fresh fluid until 100,000 miles or, with some Ford transmissions, even 150,000 miles. A lot of mechanics say that is too long and that it should be done at least every 50,000 miles. Manual transmissions require more conventional gear oil rather than automatic transmission fluid and tend to be on a different maintenance schedule, so it’s best to consult the service intervals in the owner’s manual.

Like other vital automotive fluids, transmission fluid deteriorates over time. Hard use — such as frequent stop-and-go city driving, hauling heavy loads and trailer towing — will accelerate the deterioration. That kind of driving raises the transmission’s operating temperature, and heat puts more strain on the transmission and the fluid. Unlike engine oil, which is primarily a lubricant, transmission fluid serves as both an oil and a hydraulic fluid that helps facilitate gear shifts, cools the transmission and lubricates moving parts.

If you do a lot of driving under high-stress conditions, you should check the transmission fluid level more often and have a repair shop check the condition of the fluid. Transmission fluid often is red but can come in other colors, and as it deteriorates it tends to turn darker. It may also acquire a burned odor that could indicate it needs to be changed or that the transmission is developing mechanical problems.

How to Check Your Transmission Fluid

Many modern cars don’t have a transmission fluid dipstick, and automakers instead recommend a service center check the fluid level because it’s often done from underneath the car. Use your car’s owner’s manual to determine if the transmission has a dipstick that’s easily accessible; if it does, here are a few tips when checking the fluid:

1. Use your owner’s manual to find the recommended procedure for checking your transmission fluid.

2. Park the vehicle on a level surface for the most accurate reading.

3. Be cautious of engine cooling fans that may continue to run after the engine is off, as well as hot engine components; many cars recommend that the engine and transmission be at operating temperature when checking the transmission fluid.

4. Determine if the fluid is checked with the engine running or off. This can vary from car to car and will affect accuracy.

5. Some cars recommend moving the gear selector into each gear for a few seconds before checking the fluid; always return to Park or Neutral and apply the parking brake before getting out of the vehicle.

6. Identify the transmission dipstick handle, which is typically brightly colored; again, your owner’s manual will help you find it.

7. Remove the dipstick, being cautious not to spill any fluid on hot engine or exhaust parts, and wipe off the dipstick using a clean rag.

8. Reinsert the dipstick, then remove the dipstick to check the fluid level, which should be between the low and full marks.

9. If you have a leak and need to refill, make sure you use the recommended transmission fluid, fill it to the appropriate level and get the leak fixed as soon as possible.

10. Reinstall the dipstick when done.

Another indication that the transmission fluid needs changing is if there are particles or other debris in it. When you take your vehicle in for an oil change or other routine service, the repair facility may urge you to pay for a transmission fluid change or flush. Even if they can show you that the fluid is darker than original, that might not mean you need fresh fluid right now. Step back, check the maintenance schedule in your owner’s manual and see what the vehicle manufacturer recommends before you decide. This also will give you time to shop for the best price.

Many repair shops use flush systems that force out the old fluid and pump in new fluid rather than letting the old fluid simply drain out. Though that sounds good, some manufacturers say you shouldn’t do that (Honda is one; there are others), so you need to know this before you agree to a flush. Look in your owner’s manual. Some manufacturers, such as Honda, also call for their own type of automatic transmission fluid and warn that using other types could cause damage. Moreover, some automatic transmissions have filters that should be cleaned or replaced when the fluid is changed. Make sure the repair facility is using the correct fluid and procedures for your vehicle.

If you have never changed the transmission fluid in your vehicle and have more than 100,000 miles on the odometer, should you change it now? We have seen mixed opinions on this, with some mechanics suggesting you should just leave well enough alone if you aren’t having shifting problems. Adding fuel to this theory are stories about older transmissions failing shortly after they finally received fresh fluid.

We have a hard time accepting that fresh fluid causes transmission failure, so our inclination would be to have it done if you’re planning on keeping the vehicle a few years or longer. However, fresh fluid is not a cure for gears slipping, rough shifting or other mechanical problems, so don’t expect a fluid change to be a magic elixir.