Submitted by RJ Milnes; Not long ago we had a customer come into the shop stating that the check engine light was on in their vehicle and had been for a long time. We quizzed the customer and come to find out the customer thought the light was on because they needed a 60k service. The vehicle was due for its bi-annual smog inspection and the light needed to be off before the vehicle would pass the inspection. Part of the smog inspection is a check of any diagnostic trouble codes stored in the vehicle’s control module (main brain for the engine). We performed a pre smog inspection to see what the tail pipe emissions were and to check the other emission components on the vehicle with out reporting a possible failure to the state. The vehicle failed the inspection due to a p0135 code. The code is an Oxygen Sensor Heater performance problem. I started to investigate by checking for power and ground to the sensor and I found that both were ok. The problem was in the heater portion of the sensor. The oxygen sensor has a very important job. It reports to the “main brain” the oxygen content in the exhaust system. If there is low oxygen content in the exhaust system, the engine is receiving too much fuel. If there is high oxygen content then the engine is not receiving enough fuel. If the engine receives too much fuel the vehicle’s fuel mileage can be greatly reduced and catalytic converter damage along with damage to other engine components can occur. If the engine does not receive enough fuel the same damage can occur. The Oxygen Sensor Heater is there to aid the warm up process of the sensor and help it to start working faster as it does not function when cold. We advised the customer of our findings and they decided to repair the vehicle. This vehicle has a total of four sensors and all are of the same age. It is always best to replace all four sensors if possible or feasible due to there age. It is kind of like if you installed four light bulbs at the same time and one burns out than the other three are soon to follow. So we began to remove the sensors and two were replaced without any concerns. When replacing the other two sensors one was replaced with a great amount of effort. The last sensor was very stubborn and was removed with a great amount of resistance, the sensor came out, but when it was removed the Exhaust Manifold threads were damaged beyond repair. We tried everything to repair the threads on the manifold but we were unsuccessful. The manifold needed to be replaced. Long story short the manifold was replaced along with all four sensors. The vehicle passed the smog inspection with flying colors. The customer was happy but the repair bill was almost $3,000. If the customer was educated about that little yellow light than they could have brought there vehicle in before there was extensive damage. Always remember that when the check engine or service engine soon light comes on you should have it checked as soon as possible! It could save you a headache and help the wallet!