Q: O2 sensor failed

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I let my truck sit over the weekend and it won't start. The oil level is fine, but the light did come on, telling me to change it, before I parked it. I am also losing coolant.

Hi there. A p0420 usually means the catalytic convert is not performing as well as it needs to. Over time, the catalytic converters fill up with carbon and and will not filter out the exhaust gas as efficiently as they should, which means a replacement is needed. I would recommend having a technician, like one from YourMechanic, inspect for any error codes and replace the catalytic converter if necessary.

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