In this section, we will cover some common service issues relating to emissions control components. The tips that follow may help you suggest the right part for some of the more common emissions systems failures, particularly for DIY or less-experienced customers. These tips may help you guide your customer to replace the right part the first time instead of returning a perfectly good part under warranty, when the new part does not fix the problem on the vehicle, due to misdiagnoses. Misdiagnoses of emissions issues and the replacement of the incorrect part is very common.
As a vehicle ages, all the emissions control components gradually wear and deteriorate. By the time a customer notices an actual symptom, there may be a combination of failures in different systems. On OBD II vehicles, a trouble code indicating the failure of a particular component or system may actually be caused by a failure in an apparently unrelated system. This often leads to misdiagnosis and the replacement of the wrong part.
A systematic visual inspection should be the first step in any diagnostic process. Very often, a loose connection or a cracked, missing, or disconnected vacuum hose or component can be spotted before any diagnostic time is invested by the technician. For example, the hairline crack in the exhaust manifold shown on the right can set false O2 sensor trouble codes, damage the catalytic converter, and cause the vehicle to run rich.
A Thorough Visual Inspection Can Save Diagnostic Time
The Emissions Decal Can Help to Identify Emissions Parts and Route Vacuum Lines
The Emissions Decal
It can be difficult to identify emissions control components and their correct vacuum connections, especially on older vehicles that may have a dense maze of interconnected vacuum switches and actuators. Refer to the under-hood emissions decal to help identify components and correctly route vacuum lines. Refer to the cataloging section on how to locate components that may identified by an unfamiliar abbreviation on the decal.