Check engine light

volvo
s40

#1

I replaced the oxygen sensor after getting the computer readout (front sensor bad)but after many restarts the check engine light did not reset. I had another computer read out and the same oxygen sensor I replaced is coming up as bad. Gas mileage seems bad; can the new one be a bad sensor or?


#2

Its best to have the computer reset, not just try to get the required number of drive cycles in. BTW, number of restarts does not count.


#3

There is no OBD code that says any O2 sensor is bad.
There are OBD codes that say that the O2 sensor is reading too lean or too rich or too inactive. Sometimes, it is the O2 sensor that is bad. Sometimes the wiring to the O2 sensor is bad, but most times, the O2 sensor is just trying to tell you about some other problem with the engine or engine management. You have fell into the trap of shooting the messenger. Now, why don’t you start looking for the actual problem that the O2 sensor is detecting. What are the exact codes that are present.


#4

Try disconnecting your battery for a 1/2 hour. If that doesn’t work, disconnect it overnight…

If that doesn’t work, welcome to the club of CEL drivers who’s CEL lights have been on for a LONG time but they somehow manage to get through another day…


#5

I hope you replaced the right oxygen sensor. This car has two, one before the catalytic converter and one after. The one before the catalytic converter, actually right in the exhaust manifold near the collector flange, is the one that reads the engine being rich or lean. The second one is strictly emissions, and only monitors the efficiency of the catalytic converter.

Also, tardis is absolutely correct that the code is for a circuit problem, not necessarily a sensor problem. What is the code? If the code has not reset after a couple of weeks of normal driving, the chances are the new sensor did not fix the original problem.It’s time for a proper diagnosis with a proper scan tool, not just a code reader.


#6

I did replace the front one which the code reader indicated was not functioning properly. If it wasn’t actually the sensor and the gas mileage is poor where would the problem likely be? This is my daughter’s car and she reports getting about 16 miles gallon driving around Boston.


#7

Boston in the winter, that’s not too bad…