P0135 code basic troubleshooting refers to the oxygen sensor heater circuit. As most of you know already, oxygen sensors are hooked up in the engine exhaust system in 2 locations; at the exhaust manifold and after the catalytic converter (CAT). In the first location, it is use to give signal to the engine computer so it can control engine fuel mixture. This is done by monitoring the amount of oxygen present in the exhaust gases by monitoring the equivalent voltage generated. When in operation, there is little time required to make the oxygen sensor work since the sensor’s probe requires some heating up. When that set temperature is attained, the oxygen sensor begins to work and it is called “close loop” cycle. That cycle is the time when the voltage signal of the oxygen sensor began switching between 0.1v to 0.9v. This signal reading is used by the engine computer to control the fuel trim of the fuel mixture.

So how does P0135 code comes into effect? To shortened the time to reach close loop, a heater element is added to the oxygen sensor circuit. This heating element is also being monitored by the engine ecm so that if the time required to reach the close loop started to get longer, it will set a code P0135. Most of the symptoms for this code is poor fuel economy since the engine computer will have difficult time controlling the desired fuel trim. To check this code needs testing the heater element wires for continuity, open, short and resistance.

Tips to repair P0135 code:

When checking for the voltage reading of the oxygen sensor heating element, always use the wiring diagram for the appropriate oxygen sensor circuit. In one P0135 diagnosis I saw on Ford vehicles (which can be applied to other types of vehicles), troubleshooting was done by tracing the engine computer wires. using a computer wiring diagram, you can probe the 4 pin terminals of the oxygen heater circuit with a voltmeter when the ignition key is on. If one of the wires does not show a reading, it usually leads you to the fix. If necessary, you can unplug the engine computer itself to verify the voltage readings.

Always follow the manufacturer’s P0135 troubleshooting guide when to replace the oxygen sensor because the switching voltage reading will slow down when the sensor has accumulated too many miles of operation. This is included in the service if you are a member of ATS.