1999 Safari Van - code P0420

Hello, can someone please advise me as to what code P0420 may indicate? The Service engine light is on and the engine seems to run ok. There are no other codes present.
Regards

DTC P0420 is "Catalyst efficiency below threshold bank one". When the ECM runs this DTC, there are 24 conditions that must be met before the DTC will complete its test. If it detects a problem, the DTC will be set and the SES light will come on. Due to the complexity of this DTC, it may or may not mean that the catalytic converter is performing properly. On some ECM's, the parameters for setting the DTC are too narrow, which means the DTC might be set when the converter is actually within specifications. GM has a Technical Service Bulletin that involves widening the parameters on certain vehicles, check with the dealer to see if yours qualifies.

As a vehicle is driven, oxygen sensor reaction time slows down, the more miles, the slower it gets. One of the things the DTC checks for is the reaction time between pre and post catalyst oxygen sensors when fuel mixture is changed by the ECM during the test. P0420 can be set if the oxygen sensors reaction time is slower than it should be. The sensor output voltage might be within specifications, but the ECM has no way of knowing if the sensor reacts at the proper speed, so it has to assume reaction speed is ok. As long as the sensor output voltage is within specs, no DTC will be set for the oxygen sensor. The only way to measure reaction time is with an enhanced scan tool that can "dial in" to the oxygen sensors and display the wave pattern of it.

Clear the DTC's and reset the OBDII, (must be done with a scan tool, not a code reader). Drive the vehicle, if the SES light comes on again, have a qualified technician diagnose the system, don't just throw parts at it, unless of course, money is no object.

If the vehicle has 100,000 miles or more, and the oxygen sensors have not been replaced, they're due for replacement. Replacing them MIGHT satisfy the ECM when it runs the test.

Thank you for your reply and your expertise. I do not have a proper scan tool only a code reader that can clear codes. I have to take it into a qualified garage. One of the oxygen sensors were replaced 91,000 miles ago and the catalytic converter was changed 87,000 miles ago. The van has 313,000 miles on it. Once I have the problem solved I will post the results. Sorry for the delay in my response. Does anyone know how many O2 sensors are on this particular vehicle? Thanks again.

1999 Safari has three sensors. One in each bank of the "Y" pipe, and one after the converter. All three are serviced from underneath the vehicle. Direct fit and universal sensors are available, do yourself a BIG favor and get the direct fit sensors.

I'll agree with the Master. The O² sensors are best replaced with OEM style parts with the proper connectors already attached. I'd also suggest applying sufficient antiseize compound when installing a sensor, new or old. The paltry coating usually supplied with a replacement sensor is often inadequate. Just be very careful to keep compound or any other contaminants off the sensor tip shield.

You can perform a test of the sensors without a scanner, but it is cumbersome, and they need to be removed for testing. If you remove a sensor, connect a voltmeter to the sensor output leads (not the heater leads) and place the sensor tip in the flame cone of a propane torch. A propane flame is almost completely devoid of oxygen, and should enable the sensor to generate a signal of almost 1.0V once it is heated to above 600ºF. If the sensor does not generate at least 0.85V (850 mv) it may be weak. Also, the sensor voltage should decrease to under 0.20V (200 mV) within a couple seconds when removed from the flame and exposed to air. If the sensor output signal does not react quickly to the flame/no flame condition it may be "lazy" due to contamination or deterioration.

This is not the method most commonly used by automotive technicians to diagnose a sensor, since using the scanner is so much easier. However, this is similar to the method used by Bosch and NGK for testing sensors in production.

http://72.19.213.157/files/OxygenSensorBenchTest.jpg

As previously advised, this is not the only potential cause for generating the error, but may help identify a weak or failed sensor as part of the diagnosis. This method also doesn't identify any other conditions which might be causing poor sensor performance, but can prove whether a sensor is capable of generating adequate signal and reacting quickly to oxygen as it should.

While you're under the vehicle, fighting the old sensors out of their bungs, inspect the exhaust system upstream of the sensors for damage or leaks, like rust holes, a split catalytic converter case, failed manifold flange gaskets, or other potential air leaks into the exhaust system.