Hello guys. I have 147,000 Km on my 2002 V6 and have not replaced my O2 sensors yet. Should I replace as precaution or wait until the check engine light comes on?

I would wait until they actually go bad and turn on the check engine light. Your car's performance is not usually affected in any noticable way when an O2 sensor goes out, so it's not anything that would leave you stranded should it go bad. The only thing it controls is emissions, so in one of those states that are strict about that kind of stuff, you wouldn't be able to pass inspection until you got it fixed. Otherwise it's not really a major issue.

Thanks very much

Jean,

O' contraire...you do want to replace an O2 sensor on scheduled replacement. New Bosch replacement sensors can increase your fuel mileage, so we can run the math to see how long before the sensor pays for itself saving 3 miles per gallon at $4.10 per gallon.

An O2 sensor slowly loses sensitivity over miles of use, so you likely are not getting full performance. Bosch and Hyundai recommend replacing at 100k miles/160k kilometers. I am not recommending replacement early, just replacement on schedule.

With a working O2 sensor set (you have 4 on that engine) your engine runs in an 'open loop' cycle where the ECU is receiving information and updating your fuel management hundreds of times per second. If even 1 sensor fails, your car defaults to a factory setting of closed loop...it runs using default values in a 'limp-home' mode. Which sucks the gas.

We all have limited dollars, so the O2 sensors that fail first are Bank 1 Sensor 1 and Bank 2 Sensor 1. These are all I would replace without a Check Engine Light.

Also, the dealer will charge you 25% more for the same part as Autozone or Checker Auto.

Byron