Car buying guide
Easy way to find out engine & catalytic converter health
We all know catalytic converter is expensive. One easy way to find out catalytic converter status and engine health is to use the OBD2 scanner and check upstream and downstream O2 sensors. Upstream O2 sensor should have sine wave and downstream O2 sensor should show rather flat line between 0.6~0.9V ranges.

This is the result of a recent used car inspection. The 2005 Hyundai NF Sonata vehicle has 210,000 km. The car seems to be in very good condition. If O2 graphs look like the one in the picture, we could tentatively conclude that the engine is in very good condition.
O2 1x1 is Bank 1 oxygen sensor 1 with normal sine wave. O2 1x2 looks like a horizon between 0.6 and 0.7V. Very normal. This means that the engine is good and the catalytic converter is healthy. If the graph of O2 1x2 or O2 2x2 sensor looks like sine wave, then either catalytic converter or O2 sensors are not in good condition.

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