Here is the definition for code 431.
P0431 MITSUBISHI Description
The Engine Control Module (ECM
monitors the switching frequency ratio of heated oxygen sensors 1
(front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).
three way catalyst converter (Manifold) with high oxygen storage
capacity will indicate a low switching frequency of heated oxygen sensor
2. As oxygen storage capacity decreases, the heated oxygen sensor 2
switching frequency will increase.
When the frequency ratio of
heated oxygen sensors 1 and 2 approaches a specified limit value, the
three way catalyst malfunction is diagnosed. The problem could be the catalytic converter, one of the O2 sensors, or an air leak in the exhaust system.
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