Repair Info - Emissions Repair Explained

Oxygen (O2) Sensor Testing

Emissions repair technicians should be in the habit of checking O2 sensor performance on every vehicle they work on. O2 sensors are critical to controlling emissions, and it is very common for O2 sensors to wear out and fail.

To accurately evaluate the performance of an O2 sensor, the technician must determine the voltage range, response time, and number of cross counts. To do this, the technician will need to perform at least two tests: a snap-throttle test, and a cross-count test. Both use a digital storage oscilloscope (DSO) to monitor voltage over time.

Test Condition Specification

Table 1: O2 Sensor Performance Criteria.
maximum voltage
when forced rich		 greater than 900 mV
minimum voltage
when forced lean		 less than 100 mV
maximum response time
from lean to rich		 less than 100 ms between 300 mV and 600 mV
maximum response time
from rich to lean		 less than 100 ms between 600 mV and 300 mV
cross counts
at 2500 rpm		 at least 5 cross counts in a 10 second period