Expert: Robert Summers - 11/10/2008

Question
while driving my truck one day the service engine soon light came on,I took my truck to a local auto parts store to have the code scanned;it turns out to be a O2 sensor code.I replaced the front two sensors and had the code reset.After driving the truck for a short period of time the light came back on.Again I go to the auto parts store and read the code again and its again an O2 sensor.I now replace the other two sensors,reset the code, drive for a short period of time and the light comes on again.What can be the problem, here's only four sensors! The truck only has 90,000 miles, all of which I have put on it.I know how the truck has been cared for and wish to hold onto it for another 10 years. Thanks for your consideration in advance!

Answer
Hi Eugene....it may not have been the sensors at all...thats the problem with a code reader it can only give you a code not what causes it...install a scan tool and check the readings from the o2s. if they are within spec your o2s are ok...you may have a problem in the circut to the o2...check the connections for corrosion, check the MAP sensor readings to be sure that you don't have a rich/lean problem throwing the o2 out of wack. the scan tool will also tell you which 02 is reading wrong it will say bank1 sensor 1, bank 1 sensor2 or bank 2 sensor 1 bank 2 sensor2 ... sensor 1s will be upstream and sensors 2 will be downstream closest to the converters. check and see if you have a bad converter, if it is downstream sensor error... if an upstream sensor it could be a fuel problem such as bad injector... if you can't figgure it out find a shop that has someone qualifed to diagnose the problem...hope this helps