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You are driving down the interstate on the way to visit an old friend. While you're listening to the radio and enjoying the open highway you suddenly feel something a little strange, perhaps like a slight engine surge. It does not happen again over the next half hour or so- you dismiss it as your imagination.

A little further along in your journey, you feel it again- this time it is accompanied by your check engine light illuminating. You feel a bit concerned, but the car isn't driving too badly, it just has this very occasional odd engine surge. Naturally, you decide to have it checked as soon as you get to where you are going. Here's what may be found*

As Audi's A4 and A6 models are starting to become a few years old, there are few problems that are being experienced with their transmissions. By far, the most common problem that is being seen at this point in time is an issue with the transmission's torque converter.

The problem typically starts with a check engine light coming on. While sometimes the actual drivability symptom is subtle, your vehicle's computer is able to detect it by comparing engine RPM with transmission input shaft RPM.

Often times, in addition to the light coming on, one will also notice an increase or fluctuation of RPM's at speeds above 45 miles per hour or so.

If access to a scan toll is available, either a VAG or Snap-on type with the correct European vehicle software, codes can be retrieved from the vehicle's computer in most cases.

Some of the codes that are normally found in the system include a code 18032 which is a MIL light request. This code simply means that the vehicle's computer has detected a problem and has illuminated the malfunction indicator light to let the driver know that something is amiss. This is a not so subtle warning to get the vehicle checked out.

The next two codes give a better indication of what the problem actually is. The first code that is often seen is a code 17125- torque converter clutch circuit performance. This code is set by the computer commanding converter clutch application but not seeing the appropriate drop in RPM that is associated with proper converter apply. Quite simply, the torque converter's clutch is slipping.

The second code that is usually observed is an OBD P0741- torque converter clutch stuck off. Similar to the previous code, excessive converter slippage is seen by the vehicle's computer. This code, however, usually results in the computer disabling the converter clutch's application altogether in an attempt to protect the transmission from further damage. All of this is usually going to require replacement of the torque converter.

Here's the nuts and bolts technical explanation of the root causes: ZF and Audi's original converter design utilized two rubber seals inside of the torque converter that were of questionable quality.

The two seals are responsible for sealing the hydraulic pressure that allows the torque converter clutch to engage and hold. One of the seals is a "lip" type that seals on the transmission input shaft and the other "o" ring type seal seals against the piston.

The material originally used was not up to the task when it came to dealing with the tremendous heat and pressure that occurs inside of a torque converter. Although Audi did release a second design input shaft lip seal, failures have been observed with those as well.

When the hydraulic pressure that is required to allow converter clutch application cannot be contained because of the faulty seals, the clutch will start to slip and usually destroy itself before too long.

When looking for a replacement converter to purchase from a remanufacturer, it is often helpful to ask them a few questions about their rebuilding procedures.

First off, you will want to find out if they are installing the latest design level seals as part of their torque converter rebuild- using original type seals will work for a while but will eventually lead to another premature failure. There now aftermarket replacements for these components that seem to be far superior to the factory parts.

Secondly, you will surely want to ask if the clutch itself is getting replaced. Believe it or not, this is not a given with some suppliers. The clutches in these particular units are of a multi-disk configuration. Replacing this entire multi disk set up is quite labor intensive and requires not only special tools but also a high level of skill- make sure your converter rebuilding company has both!

Naturally you will also want your rebuilder to replace any other components that normally wear out such as washers, bearings, one way clutches, and spline inserts. Most of the quality converter manufacturers will also pressure test and dynamically balance the unit once it is in the final stage of the rebuilding process.

Taking the time to find out that the job is getting done correctly is absolutely critical. As with any type of rebuilt or remanufactured automotive component, there can be varying levels of quality. With over 11 hours of labor involved in removing and reinstalling your A4 or A6's ZF5HP19 FLA transmission to replace the converter, it is surely a job that you will not want to do or have done more than once.