Why is my Vauxhall Insignia experiencing poor fuel efficiency?

I have noticed that my Vauxhall Insignia's fuel efficiency has decreased significantly recently. What could be causing this issue and what steps can I take to improve it?

Have you checked your tires for proper inflation? Underinflated tires can cause decreased fuel efficiency. Make sure to check your tire pressure regularly.

You may need to change your engine oil and air filter. Dirty oil and a clogged air filter can impact your car's fuel efficiency. Consider getting a tune-up as well.

Another possible cause could be a malfunctioning oxygen sensor. This can significantly affect your car's fuel efficiency. It's best to take your car to a mechanic for proper diagnosis and repair.