Julian the hooligan

The engine light came on. My mechanic said it was the oxygen sensor and ordered one. I asked if the car was still okay to drive and he said "Yes, just not too hard and stop if the light starts blinking." Four days later the sensor came in. On the way to the garage the engine light began blinking and the car started to shudder. I got the car to the garage. The sensor was replaced but the car still was not firing correctly. He suspected it might be the plugs but that wasn't the problem. He now assures me it's a valve. Does this makes sense to anyone.