K 1500 Yukon Denali AWD V8-6.0L VIN U (2002)
Three-Way Catalyst Oxygen Storage Capacity
The PCM must monitor the TWC system for efficiency. In order to accomplish this, the PCM monitors the pre-catalyst and post-catalyst HO2S. When
the TWC is operating properly, the post-catalyst (2) HO2S shows significantly less activity than the pre-catalyst (1) HO2S. The TWC stores oxygen
during the normal reduction and oxidation process. The TWC releases oxygen during the normal reduction and oxidation process. The PCM calculates
the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst HO2S voltage levels.
Whenever the sensor activity of the post-catalyst (2) HO2S nears the sensor activity of the pre-catalyst (1) HO2S, the catalysts efficiency is degraded.
Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the
catalyst monitor diagnostic. Similarly, if an aftermarket catalyst does not contain the same amount of precious metal content as the original part, the
correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.
Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on crankshaft rotational velocity, aka reference period, variations. The PCM determines crankshaft rotational
velocity using the Crankshaft Position (CKP) Sensor and Camshaft Position (CMP) Sensor. When a cylinder misfires, the crankshaft slows down
momentarily. By monitoring the crankshaft and CMP sensor signals, the PCM can calculate when a misfire occurs.
For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1,000-3,200 engine revolutions.
For catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions
Rough roads may cause false misfire detection. A rough road applies sudden torque variations to the drive wheels and drivetrain. This torque can
intermittently decrease the crankshaft rotational velocity. The Antilock Braking (ABS) System detects uneven speed between the vehicles wheels and
sends data via the serial data bus to the PCM to disable the misfire monitor until the rough road is no longer detected.
On automatic transmission equipped vehicles, the Torque Converter Clutch (TCC) disables whenever a misfire is detected. Disabling the TCC isolates
the engine from the rest of the drive line and minimizes the affect of the drive wheel inputs on crankshaft rotation.
When the TCC has disabled as a result of misfire detection, the TCC is re-enabled after approximately 3,200 engine revolutions if no misfire is detected.
The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to reevaluate the system.
