The following steps should always be carried out in the specified order before working in the engine compartment :

Park the vehicle on a level and stable surface.

Depress and hold the brake pedal until the engine has stopped.

Apply the handbrake firmly Braking, stopping and parking Brakes Parking .

Select the neutral position or move the selector lever to P Changing gear Changing gear Automatic gearbox see Automatic gearbox_0 Automatic gearbox see Changing gear_0 Automatic gearbox Manual gearbox see Changing gear_0 Manual gearbox .

Stop the engine and remove the key from the ignition Starting and stopping the engine Engine and ignition Ignition see Engine and ignition_6 .

Allow the engine to cool sufficiently.

Children and other people should be kept well away from the engine compartment.

Ensure that the vehicle cannot roll away unexpectedly.

WARNING
Preparing the vehicle for working in the engine compartment
Opening and closing the bonnetRefrigerant R134a Physical Data
Vehicle air conditioning systems make use of the
vaporization and condensation process. In this case, one works
with a substance which boils easily, designated as refrigerant.
The refrigerant employed is tetrafluoroethane R134a, which
bo ...
Refrigerant in A/C Service station Container, Filling
Caution
If it is suspected that chemicals were added to the
refrigerant circuit to seal leaks, do not connect the
A/C service station and do not extract the refrigerant.
...
4-Pin Heated Oxygen Sensor (HO2S), Removing and Installing
Note
Do not repair the heated oxygen sensor wires. Repairing may
result in malfunctions.
Use the faulty sensor as a guide for installing all of the
accompanying attachments, cable ties or marking bands.
...