OK a bit of background. The oxygen or lambda sensor determines the amount of air in the exhaust gases. It does this by producing a voltage inversely proportional to the amount of oxygen present. When the oxygen is low the voltage is about 1 volt and when the oxygen is high it is less than 0.1 volt. The ECU uses this feed back to continuously adjust the amount of fuel injected for combustion. These sensors only work effectively when they are hot. The manufacturers of sensors include a heater element within the sensor tip to preheat the sensor at start up when the engine is cold. Most modern oxygen sensors have four wires to them, two for the heater (often of the same color) and two for the oxygen sensor circuit. If the heater element fails the sensor is unable to work effectively when starting the engine up. In this case the sensor reports a low voltage which the ECU reads as "high oxygen in the exhaust" or a 'lean' condition and tries to compensate by injecting more fuel. When the engine eventually warms up and the exhaust gases get hot the sensor then begins to function and the ECU will reduce the amount of fuel being injected once more. It is perfectly safe to drive the vehicle. However if you are making lots of short trips in which the engine rarely gets to reach hot running conditions your fuel consumption will suffer badly as the ECU will be injecting way more fuel than you need. You will also find that your exhaust smells of un-burnt fuel and your idle maybe a bit fast and lumpy with the over rich injection cycles. Often a 'bad heater' can be just down to a bad connection due to dirt, or more often corrosion, in the plug/socket to the actual sensor and a few cycles of connecting and disconnecting it can fix it. Replacing a sensor is not catastrophically expensive, $100 or so for the part (Denso or NGK) but that can rise if the garage also fits it.
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po305 show fault, why?
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