| The heater is intended to get the oxygen sensor working as soon as possible after you start the car. However, on my 4 cyl Volvo, when I had a problem with the circuit to the heater, under light load, my sensor would cool down enough to stop working properly and set a check engine light.
If your senser is too cool to work, or if the sensor heater code is a 'hard failure' code that causes the computer to ignore the readings it is getting from the sensor and go into 'limp home' mode, that will cause your gas mileage to drop off substantially.
A heater failure is probably in the sensor, but in my Volvo it wasn't, so I would:
1) Check the power line to the oxygen sensor heater at the plug (under the plastic cover on top of your engine I think and make sure you have power there.
2) If you have power to your heaters, then you are looking at another expense, because the next step is new oxygen sensors. You could replace just one, but at 98k mi, I would do them as a matched set. The good news is that the price of sensors has come down since Bosch moved their operations to the middle east and the other vendors have matched the new lower price. They are still over $100 each. I presume there are 4 on an e46 like there are on an e36. I have not done an e46, but on other BMWs, all you need is an end wrench and a 10mm socket and a pair of ramps to replace them. Do them one at a time so you don't cross the wires! The wires are strung so that they come out different lengths so it is hard to cross them, but it would be a bugger if you did cross them because the car would not run well and it would be tough to figure out why.  |