The OBD code specifies which sensor's signal the ECM doesn't like, and it
also specifies why the computer doesn't like it. What was the OBD code?
While it's possible for an upstream oxygen sensor to cause performance
issues, that's very rare on Hyundai. Again, I'm curious to know the
trouble code, but my gut instinct is that your performace issue is
elsewhere, such as low fuel pressure or worn plugs/arcing wires.
The downstream oxygen sensor cannot cause performance problems. Its sole
function is for the ECM to compare it's signal to the upstream sensor for
the purpose of monitoring catalyst efficiency.
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