1997 Nissan Sentra

Dear Doctor: I have a 1997 Nissan Sentra with the 1.6-liter motor. The car has 170,000 miles and runs great. The problem is the “service engine soon” light had been going off-and-on for the past year, but now it’s on all of the time. I went to the AutoZone store and they said it was the downstream oxygen sensor. I changed the sensor, but two days later the light came back on. I took the car to a mechanic and he said codes P0440 and P0446 were showing for the evaporator system, canister purge valve. I did not change them yet. I changed the gas cap. My car is due for inspection soon. Can you help me with this problem? Al



Dear Al:
You need to find a qualified technician who understands the evaporator system and has a smoke machine to troubleshoot the system. You could have a bad vent solenoid, faulty charcoal canister, even a rotted gas fill tube. As for the oxygen sensor code, the circuit needs to be checked, not just have the oxygen sensor replaced. A bad wire connection or even a faulty computer can be at fault here. The vehicle needs to be checked fully.

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