Assuming that it is not an additional O2 sensor, then it must surely be a exhaust gas temperature sensor. The ECU would most likely use this to determine if the catalytic converter has reached its normal operating temperature.
.....maybe??
Jason.
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1992 Charcoal Grey SVX - 165,000km
1992 Ruby Red SVX - 165,000km
....now with sequential VIN numbers! 
1986 Holden (Isuzu) Piazza Turbo - 225,000km
1993 Mercedes W140 S500 Coupe - 153,000km
1990 Mercedes W124 300D - 313,000km
EG33 + 4EAT + VTD AWD = SVX 
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