Infiniti QX4 2001 increased gas consumption

[alfullilove]

All of a sudden my 2001 QX4 has almost doubled its gas usage. It has gone from about 350 miles to about 200 miles per tank of gas. I just got the 120,000 service and feul injection service as recommended by the service department. The gas usage is still the same--about 200 miles per tank of gas. Are there any suggestions as to what the problem may be?

[goody90q45]

Is the check engine light on? Have the oxygen sensors ever been replaced?

[alfullilove]

No engine lights are on. What are oxygen sensors? Would that have been replaced in the 120,000 miles service.

[goody90q45]

Your QX4 has four O2 sensors and chances are pretty good that they didn't get replaced during a tuneup. I'm guessing that they would run you about $100/ea installed. O2 sensors are on the exhaust manifold/exhaust pipe and measure/balance the fuel/air mixture. 120K miles is close to the life of the sensors so it wouldn't hurt to go back to the shop and have them test all 4.

[Q451990]

If you replace the sensors, I would only worry about the front sensors (the ones before the cats) since they are the only ones that are used by the ECU to adjust the fuel mixture. www.sparkplugs.com is a good source for them if you decide to go DIY on this.

Heath

[alfullilove]

I talked with the mechanic. The vehicle was placed on a machine to locate the problem, but nothing was found. An engine light would be on if the oxygen sensors are bad.

Any more suggestions?

[goody90q45]

It's not necessarily true that a dead or dying O2 sensor will activate the CEL. Is the engine idling smoothly? If not it could be any number of issues.

If it is idling good then what kind of checks did your mechanic perform? Did he/she just plug in a code reader and find no codes, or hook up to a computer display that shows the O2 sensors oscillating under acceleration, or physically measure the resistance of each of the 4 sensors at the connector?