Diagnostic trouble code (DTC) information

Condition 

Fig 1: Identifying Three-Way Catalytic Converter (TWC)

Courtesy of VOLVO CARS OF NORTH AMERICA.

The control module continuously checks three-way catalytic converter (TWC) effectiveness by comparing signals from the rear heated oxygen sensor (HO2S) with the front heated oxygen sensor (HO2S). Diagnostic trouble code (DTC) ECM-4901 is stored if three-way catalytic converter (TWC) efficiency deviates too much.

Substitute Value 

Rear heated oxygen sensor (HO2S) is disabled.

Possible Source 

• misfiring
• air leakage in the intake system
• air leakage in the exhaust system
• faulty fuel pressure?
• uneven compression?
• defective three-way catalytic converter (TWC)

Fault symptom[s] 

• malfunction indicator lamp (MIL) lit