Check Engine Light Code P0420
Catalyst System Efficiency Below Threshold (Bank 1)
How does a P0420 code trigger the check engine light? This code will trigger the check engine light as follows:
1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic
converter to reduce emissions. There is one O2
(oxygen) sensor in front of the catalytic converter (called
upstream), and one behind it (downstream). When the engine is warmed
up and running normally the upstream oxygen sensor reading fluctuates
and the downstream one is fairly steady. In the case where both
readings are fairly steady, a P0420 check engine light code is set.
Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
To diagnose a P0420 trouble code we begin by checking for exhaust
leaks under the vehicle. Then use an oscilliscope to check the waveforms
of both upstream and downstream O2 sensors. The front reading should
vary, the rear should be steady. Replace as necessary. Check the
rear Heated O2 Sensor. Check the catalytic converter, replace if
necessary. If necessary, check for wiring and component concerns.
See our article "Automotive
Circuit Testing 101", if you need more assistance with this.
Remember to refer to an model-specific repair manual for proper
Done With P0420 Page - Return To Generic OBD II Code List
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