Shouldn't the mechanic check the exhaust system before saying I need a catalytic converter?
P0420 Catalyst sytem efficiency below threshold (Bank 1)
Yes they should , this verifies the condition, The purpose of the Check engine light is to protect the catalytic convertor, Especially OBD 2, The difference between OBD2 and OBD1 is OBD2 has catalytic monitors.P0420 means the catalytic has degraded, A well maintained engine , that has suddenly developed a vacuum leak giving the engine false air can and does set a P0420 with also a lean running code.Repairing the leak can fix and restore the engine fuel trim , There are cases when P0420 occurs and the catalytic is only 5% worn , will pass emission test test but fail on the function test due to the code appearring.When it comes to P0420 either the catalytic has degraded or something is affecting the catalytic from performing its job.
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