A blinking check engine light sees a problem in the emission/engine/transmission that is serious and the vehicle should not really be driven. If the Check Engine Light/Service Engine Soon light illuminates while you are driving, the car's on board computer sees a fault in the engines management system that is causing an excess amount of pollutants to be emitted from your car. This could result in engine damage if left neglected. Some problems that can cause the Check Engine light to stay on may take two "drive cycles' to complete before illuminating the Check Engine Light. The computer sees a problem and looks for an identical set of circumstances to occur to confirm that it has indeed seen a problem before turning on the light. A Check Engine light that continually flashes while you are driving is serious and is a sign that the computer sees a problem that seriously risks damaging the catalytic converter which may result in an expensive repair or even may cause a fire! Diagnostic codes are stored in the vehicle's computer and can be retrieved using a scantool that communicates with the computer in your car. We have in our Repairpal website an encyclopedia with in depth articles about computer, and computer control systems, emission and driveability problems. http://repairpal.com/check-engine-light
Customer Concern: The check engine light is on with a trouble code P0420.
Tests/Procedures: 1. Monitor the front and rear Oxygen (O2) sensors while driving at a steady cruise. If the front O2 sensor is operating normally (switching rapidly 0.1 to 0.9 volts) and has a good pattern, then look for the rear O2 sensor to be steady and not switch rich to lean.
2. It is the job of the catalytic converter to use up the oxygen to clean up the emissions. So, if the rear O2 sensor switches like the front one, the catalytic converter is bad and will need to be replaced.
Tech Tips: Use C.A.R.B. (California Air Resources Board) Certified for OBDII replacement catalytic converter with an EO (Executive Order) number.
The blinking OD(over drive) light has to do with the transmission. This requires a scanner that can scan for trans codes. A generic OBD2 code reader will not provide you with those codes.