The modern automatic transmission is one of the most complex systems at work in your car. More so than just cycling through the gears – the transmission in today’s cars works very closely with the engine to manage fuel economy, performance as well as the overall efficiency of the vehicle.

Thus, it should come as no surprise that the transmission, is amongst the most expensive items to repair if a failure occurs. Proper transmission rebuilds can cost upwards of RM 3k even for a B-Segment sedan let alone a premium flagship like the BMW 7 Series (you can triple or quadrouple that figure in case you’re wondering).

With that said, it's best to understand the basics of an automatic transmission and be able to recognise (early on) when something isn't right. This is most important if you're in the market for a used vehicle or even if you're existing vehicle is starting to drive erratically. 

What you should know about a modern automatic transmission?

In brief, an automatic transmission transmits the rotational force from the engine to the wheels, depending on the driving conditions, the transmission manipulates torque by changing gears to increase/decrease speed, whilst keeping the engine in its optimum power range.

A modern automatic transmission is made up of hundreds of moving parts and controlled by a Transmission Control Module/Unit (TCM) which is fed by a plethora of sensors that monitor engine speed, wheel speed and throttle application.

Modern cars are so efficient because the transmission and engine work very closely to regulate when gears should be shifted and just how much power should be delivered, at precisely that moment. The TCU communicates directly with the Engine Control Module/Unit (ECM) – all of which are controlled by complex software algorithms.

Not to go down a rabbit hole – but you now understand why diagnosing transmission issues isn’t easy, and issues can be mechanical, electronic and even software-related. However, the following is a step-by-step guide to knowing if the used car you're buying has a healthy transmission.

How to tell if the transmission is not good?

Well, there are telltale signs if you know where to look. In the interest of brevity, we’re only going to cover the conventional torque converter automatic which is the most prominent type of transmission in our market.

Step 1: Start the car

You can start evaluating a used car from the moment you start it up. Check how smoothly it comes to life, and if the car idles steadily. There should be no jolts and clanks coming from just underneath the car during startup. This could be a sign of the worn transmission mounts or parking pawl actuator issues.

Also read: DCT vs torque converter, which is your preferred transmission?

Once the car is warmed up and you have the front bonnet open try to get a good look at the gearbox and its surrounding areas. Traces of oil, as well as weird smells, are red flags to catch here.

Step 2: Take it for a test drive

Test driving any used car gives you a clear picture of how healthy a car is. Regardless of buying from a direct seller or dealer, it’s important that you test drive the car and get it up to speed to allow the transmission to cycle through the gears.

Watch out for the following symptoms when you test drive the vehicle:

Notice that most of the actual causes of transmission failures are related to the automatic transmission fluid (ATF) - the ATF is the lifeblood of the transmission and that's why it's so important that the ATF and filter assembly of any transmission is replaced on time. 

That said, whilst you may not be readily able to diagnose a transmission fault just by driving the car, it is important that you are able to recognise when something is not quite right. This leads us to Step 3.

Step 3: Get it inspected by a specialist

The best thing you can do before purchasing a used car is to have it inspected by a specialist or a trusted mechanic of your choosing. This is even more pertinent if you are purchasing a premium or performance car which generally has a more complex powertrain. 

Most importantly, get the car up on a hoist and inspect the general condition of the undercarriage. This will give you the clearest look at the transmission, as well as the mounts and other connecting linkages. This is the best time to check for oil leaks or residue as well. 

Also read: Are "lifetime" transmission fluids a SCAM?

Besides a visual inspection, get your vehicle diagnosed using an On-Board Diagnostics (OBD) scanner tool to check if any fault codes have been logged previously. 

Fault codes indicate that a particular system is working outside of its safe range or if there has been a failure of a component or subset system. It logs these issues using generic codes (starting with "0") and certain manufacturer-specific codes (that start with "1"). 

These are some examples of common and universal transmission-related fault codes: 

These are some of the many fault codes that can be triggered and logged by a car's OBD system. Issues can range from a faulty sensor or dirty ATF oil to something as serious as a torque converter malfunction which might require a transmission rebuild or replacement. 

And also note, that finding out about a fault code is only effective if the mechanic/specialist is able to further troubleshoot the actual part or component that requires replacement.

Issues can be and are commonly interlinked, such as in the case of code P0715 which can be also triggered by an engine temperature sensor in addition to a transmission issue. 

Conclusion

It would be impossible to cover every type of transmission issue out there, thus the intention of this article is to equip the reader with greater knowledge on how to detect and classify transmission issues and the common probable causes. 

Finding a great used car is made easier if you employ a methodical approach and do your research on a particular model. Also, if a car doesn't meet these criteria, you'll know when to walk away from the deal. 

Also read: These 4 habits will ruin your automatic transmission

If you want to skip the hassle of finding a great condition used car, simply look for a Carsome Certified vehicle. 

To attain the Carsome Certified stamp of approval, a vehicle has to first pass a 175-point inspection. Also included is a professional car reconditioning, all-inclusive, at a fixed price.

The deal also includes a 5-day money-back guarantee and a 1-year warranty.