Replacing Audi TT Coolant Temperature Sensor

As I wrote a couple days ago, the Audi TT had evidence of a bad coolant temperature sensor. Today I made the repair and took some pictures along the way. The rest of this post details the steps involved in this fix. Because being litigious is still the new hotness, the following disclaimer applies:

If you follow the instructions you see here for this repair, you could seriously damage or destroy your car, the building it is in, and/or yourself. If you are not comfortable with these possibilities take your car to an authorized, certified, notarized, and super-sized mechanic to get this repair done.

• Audi TT with bad/flakey coolant temperature sensor


• New green coolant temperature sensor (part 059-919-501-A)


• New o-ring seal for above (part N-903-168-02)


• New plastic retaining clip (part 032-121-142)


• Flat-bladed screwdriver


• Phillips-head screwdriver


• Your own fingers

Audi TT Engine

First off, this shouldn’t be attempted if the car has been run at all in the last several hours. Completing this work without getting injured requires opening up the coolant system (which is under pressure and extremely hot if the engine has run recently) and bumping around other dangerously hot parts. For my repair I let the car sit overnight and worked on about noon the next day, more than needed, but I’d just as soon not get burned.

In order, to get to the parts we want to work on we’ll first have to remove the plastic engine cover and the plastic battery cover. There are two Phillips-head screwclips that hold the engine cover in place. They unhook with a half turn counterclockwise. Pull the front of the cover up and forward and set it aside somewhere. The three screws that hold the battery cover on are normal screws. After unscrewing them (and putting them somewhere you won’t lose them), push the cover back and left a little and pull up from the back edge and it should come out easily.

Views of the coolant

temperature sensor still plugged

in and lit with a flashlight

At this point you should be able to see and get to the coolant temperature sensor. It’s a little dark, so for my work I slid a large flashlight into place to illuminate the part and to be able to see what I was doing. The sensor is on the right hand side of the engine block down beneath a few other hoses and an electrical bundle. The pictures should give you a good idea what you’re looking for.

Open and close the radiator reservoir to the left of the engine just to make sure there is no pressure in the system. Better safe than sorry.

The retaining clip (right)

that holds the temperature sensor

in (not the metal hose clamp left)

Now that we can see the part that needs replacing we need to get it out of the engine. To remove the part first we must remove the small plastic clip that holds it in. Take note at this point of the angle at which the sensor sits in place, this’ll be important when we place the new one in. Using a flat-bladed screwdriver it should be straight forward to lever the clip out. Don’t be brokenhearted if the clip snaps in half like mine did, it’s old brittle plastic and that’s why you got a new one. The screwdriver or some needlenose pliers can roust out any leftover bits if yours does snap. Just make sure those bits are out of the way, you don’t want them inside your cars coolant system.

The port for the sensor

with the sensor pulled out

Once the clip is off the sensor should pull right out easily. Some coolant will spill out, but not too much. The old O-ring may stay down in the port, you can use your fingers to fish the O-ring out. As with the plastic clip, it’s best to dispose of the O-ring rather than try to reuse it. Not having coolant spewing out of the engine on the road somewhere is worth the $1 part cost.

The sensor connector with

it’s release tab visible

There is enough slack in the cable bundle that you can pull it up to a more reasonable location to work with it. Use a flat-bladed screwdriver to push on the release tab while pulling out the old sensor. It should come out pretty easily.

Now to install the sensor the process just moves in reverse. The new green sensor will only fit into the connector one way and you should be able to feel and hear the snap when it is together.

The new sensor in the connector

Take the new O-ring seal and get it a little wet in the port and then push it down into place with your fingers. It may take a little bit of work getting it pressed all the way down in all the way around. After the O-ring seal is seated you can push the coolant temperature sensor back down into place. Remembering the angle it fit in at is key here.

While holding the sensor down in the right spot at the right angle, the new plastic retaining clip can be snapped back into place to hold the sensor in. The metal end of the sensor should be all the way underneath the clip. It is very important to make sure that this is right. If it isn’t the sensor will blow out of the port (along with a lot of coolant) when the system gets up to temperature. Pull and push on the green plastic (not the cables) to make sure there isn’t any sloppiness in how it is fitting into place.

Now that the sensor is plugged in, pushed in, clipped in and everything looks okay, it’s time to close up the hood and go for a test drive. Don’t stray too far from home in case the sensor didn’t seat correctly. The main goal is to get the engine up to temperature and make sure the coolant system can get up to pressure without blowing the sensor out and spilling coolant all over the road. This appears to be easy as long as the O-ring sealed and the clip is in the right place. Keep an eye out for coolant on the road behind you. If you do have a spectacular coolant blowout, don’t try to drive the car with no coolant, it makes the engine sad. And be mindful of the high temperatures, the normal coolant running temperature is between 80°C and 90°C, that’s 175°F and 195°F. To reiterate, doing anything around the coolant after the engine has been running for a while is ill-advised unless hot liquid burns with ethylene glycol is your idea of a good time.

So there we are, all done. Our TT made a successful 70 mile drive today with it’s new temperature sensor and all is clear and well. Safe travels.

17704 - Error in Mapped Cooling System

It’s not a month out of warranty and the TT has had it’s first ever “check engine” light incident. Using a handy tool I ordered, I was able to discover the fault for about the cost of having the dealership diagnose the problem. The message is a little cryptic, but provides a good hint as to what is awry.

17704 - Error in Mapped Cooling System (usually temp Sensor or Thermostat)
P1296 - 35-10 - - - Intermittent

Googling the error code provides a plethora of results discussing what causes this error. The first step in confirming the problem was to figure out if the coolant temperature sensor was actually misbehaving. I cleared the code out and then had Eileen keep an eye on the laptop’s graph for the coolant temperature sensor as we drove to some errands.

Sure enough, as the engine coolant slowly rose to operating temperature, for a couple seconds at a time the sensor would send an absurdly low reading and then return to showing the correct value. While the problem didn’t look like it was happening that often and may not be enough to trigger another fault, it’s obvious the sensor is not feeling quite right (that is unless there are chunks of ice flowing around in the coolant system).

On the way back home we stopped by Chaplin’s VW and visited their parts desk. I had the part numbers, but the parts guy knew exactly what I was after when he read the description I had on the Post-It next to the part numbers. Apparently this isn’t too uncommon a fault for the 1.8T motor that the TT shares with some A4s and a variety of VW models.

Evidently the accounting for warranty repairs between Volkswagen AG and the dealerships encourages them to make the parts for frequently needed warranty repairs cheaper. In this particular case, the parts were $9.10. There was some suggestion that the mass airflow sensor was cheaper nowadays ($60 rather than $300) for similar reasons.

It turns out there are some nice articles online at AudiWorld about replacing the flakey coolant temperature sensor that causes this. I think I may try to document the repair process myself with some more pictures. We’ll see how that goes.

Helpful SUV Driving Tips

We are always looking to contribute to the greater good. We also happen to commute into Seattle on I-90 everyday, and as it turns out there are a lot of people with large trucks and SUVs that commute into Seattle from the Eastside every day. All the large vehicle traffic makes me hopeful for the construction, pediatric medicine and farming industries, but I digress.

To help give back to society, here are some important safety tips for completing your commute on the freeway without injuring yourself and others in your large SUV or truck:

• The stripes on the road are not art
  While it might be easy to fall into the trap of thinking that the dashed white lines on the freeway are a form of abstract art, they are there for a reason. They, in fact, indicate the general direction you should be going. But wait, there’s more! As an added feature the space between the dashed lines is moderately larger than most vehicles, even your F-150, Suburban and Escalade. If we all stay between the lines we can fit more cars on the road and all get where we’re going faster and more safely. The spaces between the dashed lines are called “lanes”.

On many sections of freeway there are even helpful bumps on these stripes so you can feel when you are drifting into a lane other than your own. (More on changing lanes properly in the next bullet point.) When you feel these bumps you should direct your large truck or SUV back into your own lane; that way other people don’t have to swerve to avoid you. I realize that these sets of bumps can help you track each dollar of gasoline or diesel you burn on your way to wherever you’re going, but please use your gas gauge for this instead, for the children’s sake.

• That stick hanging out the left side of your steering column is useful
  As it turns out, that stick that is protruding from your steering column isn’t a practical joke on behalf of some GM autoworker in Mexico that constructed your “I only buy American” vehicle. When you move this stick up or down it will cause lights on the right or left side, respectively, of your vehicle to blink, alerting everyone else on the road that you intend to change from one “lane” into another. You’ll find that it’s a lot easier and safer to participate in freeway driving if we don’t all have to second guess where each other are going next.

While it is true that the likelihood of the State Patrol bothering to write a ticket for failing to signal while changing lanes is zero unless you manage to hit someone else doing it, it is still illegal and impolite. You wouldn’t want undue scratching and damage to the fancy paint your offroading truck/SUV when you hit someone who was unable to read your mind and tell you were going to drift over into their lane without signalling. So it’s for the safety of your SUV/truck and your children who want the vehicle not to be scratched as well. Think of the children!

• Keep right except to pass
  There are signs on this particular stretch of freeway that remind us all that the laws of Washington state say that if you’re not passing someone you need to keep in the rightmost lane. It happens that if everyone followed this simple rule, we all wouldn’t have to change lanes quite as much to make our way to our destination. All that lane changing and vehicles going different speeds in the same lane leads to accidents. And even in an SUV your 8 children that are along for the ride might get injured in an accident. Think of the children!

Now again, I’m fairly certain no one is actually ever ticketed for this bad behavior, despite the fact that it results in a large number of speed differential-related accidents. This doesn’t mean it isn’t illegal, impolite and dangerous to your children, you know, the ones you’re supposed to be thinking of all the time when those bad, evil terrorists are making our threat condition burnt sienna.

• Summary

Maybe some day in the enlightened future the various law enforcement agencies out there will actually bother to enforce traffic laws in addition to those related to speeding and the number of passengers in vehicles in the carpool lane. That way they could help encourage polite and safe driving behavior and reduce the number of accidents. Until that day comes, please follow these tips and stay safe, do it for the children!