hurricanesteve

are they measured by the O2 sensor(s) or predicted somehow?

In particular on my 99.5 1.8T, I can watch group 049 and field 2 shows either cat or exhaust temperature. It isn't labeled (just temperature)...climbs to 840C on easy highway driving...settles to 650C or so at idle.

anyone know where this number comes from?

Ferris TMK

EGT sensor located in the manifold.

hurricanesteve

I swapped the A4 stock manifold for the Stage 3 manifold, and I didn't run across any sensors in the stock manifold...

Is it in the cylinder head? Doesn't seem likely...

dingster

There are no sensors in the exhaust manifold.

Electron Man

...along with voltage output (variable w/O2 concentration) is an ECU input as well. This is my "guess" as to what you are monitoring.

The ECU monitors temp. of the first O2 sensor so it "knows" when to begin using the voltage input (O2 conc.) from the sensor.

IIRC, the voltage output is erratic until the O2 sensor reaches a certain temp...say ~500F. Prior to that, ECU ignores O2 sensor input.

Audiboy

that is to bring the 02 sensor up to temp quickly - mainly for improved emmission control. If you measure the two wires, you get a steady state 12V.

Although I haven't put much thought into the exhaust temp, it probably is calculated based on other typical sensors already present like the water temp, A/F ratio, 02 content before and after cat - or not necessarily 02 content, but the ECU's interpretation of the 02 voltage, and other sensors.

Rob K

Is it a 4 or 5 wire 02 sensor?

Audiboy

The Audi pre cat sensor is a 4 wire, as with most OBD2 ones. I haven't come accross a 5 wire yet