The head on my 1989 245 DL warped after overheating, causing significant contamination of the engine oil with coolant. This also resulted in engine codes for faults with the air mass meter and O2 sensors. After I replaced the head gasket, which included rebuilding the head, I got a "no fault" code "1-1-1" . Yet after running the car for about 15 to 20 minutes the check engine light came on and the engine code was for an O2 sensor fault. I immediately changed the oil, thinking that there may still be some coolant in the crankcase (coolant can cause failure of the O2 sensor) and reset the engine codes - however, the O2 fault code occurred again after running the engine for about 20 minutes. At this point I replaced the O2 sensor, but upon driving to the emission control station the check engine light came on yet again. Despite this the car passed the testing easily. Upon checking the engine codes, the same O2 sensor fault code came up. I changed the oil again and reset the codes but the same pattern repeated itself. Any idea about what may be causing this problem? Is there residual coolant in the exhaust system that is contaminating the sensor?