Excessive system backpressure can generate a converter efficiency code. Although backpressure varies with application, typically the pressure should be less than 3 psi at 2,000 rpm, and less than 1.5 psi at idle.
Using the O2 Sensor ports, check for excessive backpressure ahead of - and behind - the converter.
High backpressure behind the converter indicates a restricted muffler or resonator.
High backpressure ahead of the converter indicates a restricted converter.
High backpressure at the exhaust manifold indicates a blocked Y-connection.
Excessive backpressure will adversely affect AFR, leading to excessive emissions.