There is a problem with the heating element on the front, or pre-catalytic converter oxygen sensor.
What P0135 really means
When the car is completely warmed up, what engineers call closed loop, the front oxygen sensor, or air/fuel ratio sensor, plays a large role in fuel management. Because closed loop emissions are much lower than when the car warms up, oxygen sensors are equipped with heating elements to bring them up to operating temperature as quickly as possible. If the computer detects that too much time has elapsed between start up and oxygen sensor activity, the check engine light is turned on.
What are the symptoms of a P0135 code?
There might not be an noticeable change in the performance of the car. Normal operating temperature for an oxygen sensor will be around 600 degrees fahrenheit, whereas exhaust gas temperatures will be higher than this when the car has warmed up. Of course, the oxygen sensor will not work at peak performance, and you might see a dip in fuel economy. Carbon and other deposits will build up at a faster rate in the engine and in the catalytic converter. Oxygen sensors, when working well, help the engine run at peak efficiency.
What is the severity of a P0135 code?
Minor. There is no immediate danger to the car, nor will this code leave you stranded on the side of the road waiting for a tow. There is time to look for a good mechanic and plan a date to drop off the car.
What repairs are needed for a P0135 code?
Replace the front oxygen sensors. Though you will not be stranded on the side of the road, remember that the car is not running as efficiently, which will lead to more significant problems (and more money) down the road. Carbon build up in the engine and deposits on the catalytic converter could cost you far more money than even the most expensive sensors.
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Service article written by an ASE Master Technician