Increased HC with moderate O2 suggests what potential engine issue?

Increased hydrocarbons (HC) alongside moderate oxygen (O2) levels typically indicates an ignition misfire rather than other conditions. When there is a misfire, not all the air-fuel mixture is properly burned during the combustion cycle, leading to an increase in unburned hydrocarbons being expelled into the exhaust.

The presence of moderate oxygen suggests that there is still some air available in the exhaust stream, confirming that the air-fuel mixture was not fully combusted due to issues such as a faulty spark plug, ignition timing problems, or electrical issues that prevent complete ignition. This environmental condition directly correlates with the elevated HC readings, as the unburned fuel contributes to those levels while the moderate O2 indicates incomplete combustion rather than an overly rich or lean mixture.

Thus, identifying ignition misfire as the cause helps technicians troubleshoot engine performance and emission issues effectively.