P0491 trouble code Chrysler

What causes a P0491 trouble code in a Chrysler

Trouble code P0491 trouble code is defined as Secondary Air Injection Excess/Insufficient Air Flow. This code is triggered when the Engine Control Module (ECM) detects that there is not enough air being pumped into the exhaust system to help reduce emissions. The secondary air injection system is designed to pump fresh air into the exhaust stream to help burn off excess hydrocarbons and carbon monoxide before they reach the catalytic converter. To understand the purpose of the secondary air system and to understand how it works, read this post

The most common cause of a P0491 on a Chrysler

On some Chrysler engines, the feed pipe from the pump directs air into a chamber cast into the cylinder head and there is a port drilled from it into each individual exhaust port. Any restriction in flow to this chamber will cause a low flow code.

Check the secondary air ports

The O2 sensor won’t see the additional air if the port is clogged. And, if the cylinder head has ever been replaced with an incorrect head, it may not have the drilled port.

If the pump turns on and the air is moving through the one-way valve, but the code still appears, look for a restriction or missing ports.

The most likely causes of a P0491 trouble code

• Faulty Air Pump: The secondary air pump may fail or become clogged, preventing it from providing the necessary airflow.
• Defective Air Pump Relay: The relay that controls the air pump could be malfunctioning, preventing the pump from operating properly.
• Blocked Air Injection Hose: The hoses that carry air from the pump to the exhaust system can become blocked or damaged.
• Faulty Check Valve: The check valve in the secondary air injection system may be stuck closed, preventing air from entering the exhaust.

Secondary air pump

• Wiring Issues: Damaged or corroded wiring and connectors can cause problems with the secondary air injection system.
• ECM Malfunction: In rare cases, a problem with the Engine Control Module itself could cause a P0491 code.

How the ECM sets a P0491 code

The system uses the Oxygen sensor to detect a lean condition during cold startup. The computer calculates the proper air/fuel mixture based on engine temp and MAF sensor readings. By pumping extra air into the exhaust pipe ahead of the oxygen sensor, the system creates a lean condition, and the ECM expects to see that. Depending on the make, model, and engine, the PCM can test the system by quickly turning the pump on and off to see what readings come back from the O2 sensor.

©, 2015 Rick Muscoplat

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