Catalytic Converter Efficiency Failure

NAME OF CODE:
Catalytic Converter Efficiency Failure

WHEN MONITORED:
After engine warm up to 170°F, 180 seconds of closed loop operation, open throttle, at a speed >20 mph, with the engine at 1560 - 2460 RPM and MAP at 270 - 380 mm Hg (10.5 - 15 in Hg).

SET CONDITION:
As the converter loses the ability to store oxygen, efficiency drops, and the concentration of oxygen becomes the same downstream as upstream. The output voltage of the downstream O2S copies that of the upstream sensor with a time lag (seen by the Powertrain Control Module (PCM)) between the switching of the O2S's. The Malfunction Indicator lamp (MIL) illuminates after 2 trips and remains on, but goes out if the conditions that set the trouble code are not found on subsequent trips.

THEORY OF OPERATION:
The catalyst monitor uses oxygen sensors (O2S's) to monitor the efficiency of the converter. The dual O2S strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter. As the converter absorbs the oxygen, there is a lack of oxygen downstream of the converter and the output of the downstream O2S indicates limited activity. The downstream O2S detects oxygen content in treated gases that passed the converter. The PCM monitors lean-to-rich switches of the dual O2S's. A ratio of downstream to upstream switches closer to 0 indicates an operational catalyst, while a 1:1 ratio indicates an ineffective catalyst.

PROBABLE CAUSES:
- Catalytic converter
- Exhaust leaks upstream of catalyst
- Contaminated fuel
- Mechanical engine problem