The oxygen sensor on your Opel Astra is part of your vehicle's emissions controls system, and is responsible for monitoring -- as the name implies -- the mixture of air and gasoline used in your Astra's combustion chambers. If the oxygen sensor happens to go bad, it can both affect your engine's performance and cause you to fail your next smog check. Because of this, it is important to know where the oxygen sensor is located on your Astra so you can replace it when needed.
Position your jack under a sturdy point on your Astra's rear undercarriage -- the axle or frame is preferred -- and raise the vehicle off the ground.
Settle the Astra onto jack stands. Place wheel blocks in front of the front wheels.
Start at the exhaust pipe and work your way forward towards the engine. The oxygen sensor will be situated on your exhaust manifold either after or just before -- depending on the year and model of your Astra -- the catalytic converter. The catalytic converter, in turn, is the large metallic box attached to your exhaust manifold. The sensor will resemble a large golden plug with wires running out of the tip.