Likely same for all 4.2 V8s
The coolant temperature sensor on many modern Audis is highly unreliable. If your water temp gauge suddenly stops working, or works intermittently (e.g.: center for a while, then suddenly drops to zero, then comes back, maybe) it is VERY likely the sensor/sender is faulty.
I am assured that this sensor is not the same one that sends mixture/cold enrichment signals to the ECU, so driving with it faulty does not affect the mixture. It does mean you have less warning of impending cooling problems, however – the gauge is there for a reason!
The part costs anywhere from $11 (cheapie aftermarket) to $15 (brand name aftermarket – Meyle) to about $40 (dealer part). I bought a meyle. You also need a new rubber sealing O-ring, and should get 1 (or even 2) snap clips that secure it from coming out under hydraulic pressure. I bought mine from ECS Tuning, although I normally patronize my local indie.
The parts look like this:
Note the shape of the clip. It helps you positively ID the sensor when its buried. It also helps you understand how to remove it.
Here’s a closer-up look at the sensor itself, side view:
The O-ring pushes on the metal end and seals it in the engine plumbing.
Before we begin, you will need:
On V8 C5 cars the sensor is between the firewall and the motor, sitting vertically (it pushes down into its socket), below the air intake accordion hose – the roughly 4” hose that goes from your air box to your intake manifold, with stuff along the way.
First remove the engine cover to get better access and loosen the coolant overflow cap to release any pressure. Don’t do this with a hot motor!
Next remove the accordion and the hard tube (on the right in the pic below) running down toward the motor. They look like this, with the accordion out:
A 10mm socket makes this much easier. Simply loosen the circlamps (get them VERY loose), slide them aside, and gently separate the rubber hoses.
Now you need to find the sensor –it’s down below, and fairly hard to get at with your hands. I’ll explain how I got it out and back in.
In its native environment, it looks like this:
All you can really see is the wiring. But if you look closely you will see the telltale clip and a little green plastic. First you pull off the clip. It pulls straight out. I lot mine in the bowels of the car immediately as I pried it out – its very tight. Next I found it was far easier to remove the sensor and wiring as a unit and separate them later. Note: when you take it out, you will lose about 1 L of coolant, o be prepared with fresh coolant.
After I removed the clip, I used “gas” pliers to grab and pull the sensor straight up without damaging the wiring. The O-ring stayed in the socket, and while I fiddled to get it out, the 1L of coolant spilled out. Ah well.
I had previously seated the new O-ring on the new sensor. Now push them in place. I used a rag so the sharp edge would not cut my hand. It’s a tough space to work in. Next push the new clip (you bought one right?) onto the sensor. It pushes on exactly horizontally, and you can actually see it sliding above the shoulder of the sensor, locking it down. I pushed it on from the driver’s side – opposite of how the old one was. Much more room. See the new sensor – but no wiring – in place below.
Put the air tract back together, top up the coolant with 50/50 or 60/40 G12:distilled water, and you are, we hope, done.