MPGs on a steady decline over past 4 years

[ever_green]

I owned my 03 TL since new. its a decade old now and is giving me its best consumption ever on original sensors. all my cars (usually imports) power and efficiency improve with time. I had an acura rdx which started putting more power to the wheels year by year until it hit 60k miles. it never lost power or MPG though. I could still get 25mpg on highway with its 2 ton curb weight and nearly 200 horses at the 4 wheels just by using cruise control at 70mph. this was after 60k miles of mostly stop and go driving.

if youe o2 sensors had problems you would get a code and CEL...ECM monitors both o2 sensors (downstream and upstream) readings and if they are the same or similar it will throw p0420 code. then you can change your o2 or cat depending.

[MaxMatt]

Thanks for input ever_green!

However, with all due respect to your experience, bear in mind that the car in question is 20 y/o and has over 120K miles on it.
There is no CEL, there is only one o2 sensor - in front of the cat. It's not an OBD II car, it has a 4-speed auto transmission with no electronic control whatsoever.

I also have a 25 y/o car with over 400K miles on it and one that is over 40 y/o.

I love older cars and I do my best to keep them in top condition. However, technology advance is now faster than ever. You cannot compare a 40 y/o car with a 20 y/o one and a 20 y/o one with a 10 y/o one.

There's an informative article about o2 sensors here:
Oxygen Sensors & Emissions - Facts & Repair Advice

My sensor is a little bit overdue for replacement.

[ever_green]

I am sorry. I had no idea it was an old car. thought it was a decade old max.