Blazer is a 1997 s10 4x4 112k with 4 O2 sensors and this code saying the cat converter is bad doesn't seem like the problem. Truck has never run better although there is the occasional gas smell so it may be running a bit rich but other than that acceleration, rpms, shifting all flawless. I changed the 2 O2 sensors before the cat thinking it might solve the issue and cleared the code by disconnecting the battery but the code tripped again after 80 miles. Should I have changed all 4 sensors? Is there something else I should be checking like the Egr or Ect valves? Gas mileage has remained constant and aside from the occasional gas smell there are no symptoms other than the SES light so it's got me wondering where to look. Any suggestions or possible solutions would be greatly appreciated.

 

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

I would not have recommended changing any O2 sensors in this case until a proper diagnostic had been completed. The reason is because the O2 sensors are only used to verify what the PCM is attempting to do in checking the efficiency of the catalytic converter. If there were a problem with an O2 sensor, the PCM would have triggered a code for the O2 sensor.

Here's some light reading on the code and diagnostic steps:

[quote]CIRCUIT DESCRIPTION
This diagnostic test is designed in order to measure the efficiency of the Three-way Catalytic Converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting the levels of Hydrocarbon (HC) , Carbon Monoxide (CO) , and Oxides Of Nitrogen (NOx) to less harmful gases. The control module is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past. Once the conditions for running this Diagnostic Trouble Code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.

For example, if the control module were to command a rich mixture, the upstream Heated Oxygen Sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream Oxygen (O2) sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.

CONDITIONS FOR RUNNING THE DTC

* No active secondary AIR DTCs
* No active CMP sensor DTCs
* No active ECT sensor DTCs
* No active EGR DTCs
* No active Fuel Trim DTCs
* No active IAC DTCs
* No active IAT sensor DTCs
* No active MAF sensor DTCs
* No active MAP sensor DTCs
* No active O2 sensor DTCs
* No active transmission DTCs
* No active EVAP system DTCs
* No active TP sensor DTCs
* No active VS sensor DTCs
* No active misfire DTCs
* The engine speed is 900 RPM or more for more than 35 seconds since the last idle period.
* The engine has been running for more than 346 seconds and the long term fuel trim is stable.
* The predicted catalyst temperature is more than 475°C (887°F) .
* The system is in Closed Loop.
* The BARO is 73 kPa or more.
* The IAT is between -7 to 117°C (20-167°F) .
* The ECT is between 75-117°C (167-243°F) .
* The engine has been idling for less than 60 seconds .
* The actual engine speed is within 100 RPM of the desired idle speed.

CONDITIONS FOR SETTING THE DTC
The VCM determined that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold.

ACTION TAKEN WHEN THE DTC SETS

* The control module illuminates the Malfunction Indicator Lamp (MIL) if a failure is detecte