Assuming the exhaust leak is located between the exhaust manifold and the catalytic converter, you will need to have it repaired before your Lexus LS 400 can pass the smog check. The exhaust leak can absolutely cause an emissions failure. The fault here is two fold, in that, it posses a "functional" failure if the leak is loud enough to be heard by the smog technician, and a high emissions reading (as your vehicle has experienced) as well.
Having an exhaust leak such as this will disrupt the engine's air/fuel ratio (as it exits the engine) which is necessary for proper catalytic converter operation, typically causing high NO. The exhaust leak may also cause improper oxygen sensor readings, compounding the high emissions problem. Without correct oxygen sensor readings, the engine emission computer, can not determine how much fuel to the air/fuel mixture. Incorrect air/fuel ratio (14.7:1 is optimal), will cause force the engine to burn fuel inefficiently.
posted by SmogTips Support 04-25-2018 04:35 PM