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Toyota Paseo failed smog check after replacing Oxygen Sensor

I had a pre test for smog check done on my 1993 Toyota Paseo. My car registered as a gross polluter. Toyota recommended I replace the Oxygen Sensor. I had it replaced and it still did not pass the smog test. According to Toyota my catalytic converter tested ok. I'm now out $800.00 and not sure what might be the next logical step to take. Any suggestions would be helpful. Thank you.
Answer:

Along with the oxygen sensor there are additional sensors your Toyota Paseo has which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio). The main emission parts include the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If your Paseo's coolant temperature sensor is not operating properly (not sending the correct voltage signal to the Engine Control Module) the ECM will not be able to properly calculate how much fuel to deliver to the combustion chambers. When your Toyota's engine is cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Insuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). Your Paseo's MAF sensor measures the amount of air entering the intake manifold. This sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers. We recommend raising the bar at the Toyota dealership and holding them accountable for their diagnosis. Your Toyota Tacoma's entire emission system needs to be accurately diagnosed before simply suggesting changing the oxygen sensor. posted by SmogTips Support

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