Is Your Transmission Light On? (Common Causes and How to Fix)

Having an illuminated transmission light is bad news!

There could be several reasons for the transmission light staying on but one cause is by far the most common. Let’s look at why your transmission light is coming on and how to properly diagnose and fix the issue.

Low Transmission Fluid (Most Common)

An overheated transmission causes mechanical failures in a car.

To prevent such problems, car owners must lubricate their transmission using transmission fluid for proper gear shifts. The fluid is also responsible for keeping the system cool. Over time, it may evaporate or leak, leading to problems with the transmission.

Symptoms of Low Transmission Fluid

Car owners can look for the following signs to help them determine a low transmission fluid problem:

  • Humming noises
  • A burning smell
  • Slipping out of gears

How to Fix

There are two possible solutions that you can try:

  • Topping up the transmission fluid
  • Fixing any leaks

Topping transmission fluid ($20 to $50 per gallon) is a quick do-it-yourself task. But remember, different vehicles use different types of transmission fluid. Make sure to check the car’s owner manual for the right kind.

Remove the dipstick and wipe it clean with a cloth. Place it back in for a while and take it out once again to examine the fluid level. If it is low, you would need to top it up.

Fixing transmission fluid leaks is a bit technical, but nothing too challenging. Check the pan gasket for any loose bolts that can cause leaks. If the bolts are tight, then you may have a cracked pan gasket.

For minor leaks, you can use transmission stop leak fluid ($10 – $12). For larger cracks, you would need to replace the gasket ($20 – $25). You can do this at home and save yourself some money!

Besides low transmission fluid, dirty fluid can also cause problems with the transmission, causing the light to come on.

Check For Dirty Transmission Fluid

transmission fluid color chart

If the fluid level is okay, then you will need to check the color of the transmission fluid. A darker color is a sign that you should replace the fluid.

Remove the dipstick and wipe it with a paper towel. Transmission fluid has three different colors to help car owners determine the nature of the problem and these are:

  • Bright red: Normal
  • Dark red: Fluid needs replacing
  • Brown or black: An alarming sign

If the fluid levels are fine and the color is bright red, then the problem is something else. Having an OBD2 scanner can help identify the main problem.

A brown or black fluid color is a cause for concern as something bad may already have happened with the transmission system.

To sort this out, there are three possible solutions:

  • Changing the oil filter – $100 to $200
  • Complete transmission flush – $250 to $340
  • Replace or rebuild the whole transmission – $1,200 to $5,000

You should take your vehicle to a nearby mechanic when the transmission fluid is brown or black.

Use an OBD2 Scan Tool for Accurate Diagnosis

If the transmission fluid is okay, then you may have bigger problems on your hands.

To determine where the problem lies, an OBD2 scan tool can come in handy. An OBD2 scanner is a fantastic investment. It helps car owners identify underlying issues with their vehicles, helping them save a lot of money down the road.

An OBD2 scanner reads fault codes from different car modules that indicate the problems with the vehicle. However, there is bad news.

A cheap OBD2 scanner will only read fault codes from the engine module and can not help with transmission diagnosis. For that, you need something a little advanced but not too much. A mid-tiered OBD2 device should work great here.

Common transmission fault codes and what they mean

Here we will look at some of the most common fault codes that may come up on your OBD2 scan tool and what it means.

  • P0218: A sign of an overheated transmission. If the transmission fluid levels and colors are fine, it could be a temperature sensor issue.
  • P0613: A malfunction due to shorted wiring or processor failure. A mechanic can fix this in a jiffy, but it’s going to be an expensive repair job.
  • P0614: Incompatibility issues between the engine control module and transmission control module. A recent change in these components could be causing this problem to occur.
  • P0700: With this code, your transmission light might be accompanied by an illuminated check engine light. It could indicate a faulty transmission control module.
  • P0706: A possible issue with the transmission range sensor responsible for smooth gear shifts in a vehicle.
  • P0715: A shorted wire or a faulty input speed sensor could trigger this fault code.
  • P0720: A possible issue with the output speed sensor can cause a vehicle to have a boggy speedometer, triggering this fault code.
  • P0729-P0736: A potential gear issue requiring a transmission fluid top-up. However, if you still see the fault code after filling the fluid, you may have an underlying problem with the torque converter or corroded solenoids.

Why You Shouldn’t Ignore the Issue

An illuminated transmission light is a bad sign that requires your immediate attention. Prolonging a car’s diagnosis can complicate existing problems, further worsening the transmission. This can hike your repair/replacement costs to thousands of dollars.

An OBD2 scanner can help detect underlying issues, equipping you with enough information to head to a mechanic and fix the problem. Investing in one will save you a lot of money in the future.

Many choose DIY car repairs to save money, which is quite alright. However, transmission problems are often complicated and you don’t want your car to start having problems on the road due to improper DIY repair jobs. If you don’t know what you’re doing, it is best to let a professional have a look at it.

See Also8 Signs of Transmission Troubles

Steve Cooper

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