There are literally dozens of potential causes for a decrease in fuel economy - many of them will also cause other symptoms with the way the vehicle operates, which can be helpful in diagnosing the problem. Some of the more common ones are:

* Fuel/Intake System Restrictions - Dirty air filters, fuel filters, and fuel injectors make your engine work harder to generate the same amount of power at the wheels, thus wasting more of its own energy and reducing mileage. This is probably the most common cause of gradual fuel economy loss because, unless the problem is severe, computer controls can usually compensate such that the average driver doesn't notice any issues. This is why it's important to stick to maintenance schedules, even if it seems unnecessary at the time.

* Engine Management Problems - Modern vehicles rely heavily on computer controls to "fine-tune" the engine in real time for maximum efficiency; an assortment of sensors monitor various engine parameters and feed data to the computer that allow it to make adjustments as conditions warrant. Most of these sensors are subjected to some "hostility" during operation (high temperatures, corrosive gases, contaminants, mechanical wear, etc) and do occasionally fail, preventing the computer from correctly doing its job and consequently reducing efficiency. Similarly, the computer in turn uses some electronic and electromechanical devices to actually control the engine, which can suffer the same fate. Problems of this type will almost always turn the Check Engine light on and cause additional driveability issues (rough idle, poor acceleration, stalling, etc).

* Wheel-End Problems - Low tire pressure is another common cause of low fuel mileage. Low pressure allows a greater portion of the tire tread to contact the road, increasing friction and sapping power (it also causes accelerated and abnormal tire wear). Wheel alignment problems cause similar issues by forcing tires to "drag" across the pavement to some degree, rather than roll smoothly. Less commonly, malfunctioning braking or all-wheel drive systems can cause additional drag at one or more wheels, wasting power.

* Environmental Issues - The environment and manner in which a vehicle is operated can have a big impact on its fuel economy that you might not consider unless you stop to think about it. Your mileage may decrease in snow, for example, because you spend more time in stop-and go conditions and in low gear, or in the summer months if you drive more aggressively. Changes in overall driving habits (going from mostly highway to lots of city driving) can also play a big role.

* "Parasitic" Problems - The engine is ultimately the source of all power used by every system in the vehicle, and as such, any part of the vehicle operating in an inefficient manner has the potential to reduce fuel economy. Clutch/transmission slippage, for example, can cause a dramatic loss in fuel economy, as can regenerative braking malfunctions in hybrid vehicles. Engine accessory problems (water pumps, alternators, etc) can put fuel-wasting drag on the engine, as can internal mechanical problems of the engine itself (these types of problems usually make themselves apparent in other ways - a bad alternator will cause electrical problems or make noise, for example). Even body damage or modifications can increase wind drag, decreasing fuel economy.

One last thing to keep in mind: it is entirely possible for several causes to simultaneously contribute slight effects which all add together to cause a more pronounced problem; hence, it isn't always possible to pinpoint a single "silver bullet" that will cure poor fuel economy, especially in higher-mileage vehicles.

Typically, the oxygen (O2) sensors are replaced in pairs - due to the age and wear - assuming they were installed at the same time originally.

If you have someone read the code (free at autozone or advance auto, etc) they will normally tell you "bank 1 sensor 2" or some such. My 1999 9-3 SE has 4 O2 sensors! Autozone actually has a spot on their web page that showed me where they were all located.

Hello Marie,

In order to locate the faulty oxygen sensor, you will need a scan tool to determine which sensor is faulty. When reading the scanner you will find codes and also the location of the faulty sensor. If your vehicle has over 100,000 miles on it, it may be prudent to replace both sensors. The correct Walker part numbers for the sensors are as follows

Upstream Sensor Walker 250-24497
Downstream Sensor Walker 250-24842

If you have any further questions or concerns please contact us and we will be happy to help.

Walker Pro (www.walkerproducts.com)

take a obd2 code scanner(careucar.com) to have a check, and you will knwo what problem cause the light on.

The white smoke out the exhaust is not a positive sign - this condition may point to a leaking or blown cylinder head gasket. Have an inexpensive cooling system pressure test performed to determine the engine condition and head gasket sealing integrity.
Check the many on-line parts sources;such as www.RockAuto.com, www.autopartswarehouse.com, or www.autopartsfair.com. For example, they (RockAuto) list the cylinder head gasket from around $48.00 for your vehicle.

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Turn the crankshaft so that the #1 pistion is up all the way. You should also notice that the timing mark for the crank is lined up also. if its not lined up, turn it one more time till it is and #1 piston is up.

It could be nothing major, just a software update and everything is fine (most of the time it's fixed this way). Rarely I've heard of major transmission failure, and had to have parts or the entire transmission replaced.

See this link http://www.autopartswarehouse.com/

AutoMD.com is a public domain.

4 cylinder engines usually have two. One in front of the Cat and one after. Do you have a check engine light on?

I believe yours is the same as mine (1999 9-3 SE) so it would be 4. Autozone.com has a great diagram. If you look up O2 sensors, it will ask you to choose your car and model. Then go to the right side of the screen where it says "View Job Diagram" it shows a blow up diagram of the sensors. If you have a check engine code read- it will tell you which bank and which sensor.

Ya need to scan it and see why the check engine light is on . A lot of parts stores will do it for free . AUTO ZONE , PARTS PLUS , ECT . Otherwise it's just a shot in the dark .

Not sure who would buy used, worn out fuel injectors - these are considered as a wear and tear (throwaway / dispose) item - and are not usually rebuildable.