1998 Honda Accord Engine Trouble Codes.

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For the code P0420 you will first need to inspect your oxygen sensors for proper operation. To learn how this is done, follow the link below..

https://www.2carpros.com/how_to/how_to_test_oxygen_sensor.htm

If the oxygen sensors are OK then your delete the engine code with your scan tool. If the code comes back then you will need to replace your vehicle's catalytic converter.

For code P1259 is normally caused by a faulty vtec pressure switch or wiring to that switch. Inspect the wiring and if necessary replace the switch.

Let me know the results and I will follow up.

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