Service Engine Light

Asked by Oct 30, 2016 at 06:23 PM about the 2007 Nissan Altima 2.5 S

Question type: Maintenance & Repair

I just got a used 2007 Altima. The service engine
light is on again. We just had both catalytic
converters changed, oil change, and there was a
vacuum hose replaced. Now the light has come on
again. I took it to the auto parts store and the code
this time is: PO420. Can you help?

2 Answers


Read through the possible causes. http://www.engine- good shop will be able to diagnose the exact issue. Sounds like your not so lucky.


Did the shop clear the computer after the catalytic converters were replaced? They should have. Did they put the correct converters on your vehicle? If you have California Emissions and they put Federal Emissions converters on it that will do that. Did they properly warm up the new converters before they gave the vehicle back to you? If not, damage will occur to the new converters. Putting on the wrong converters or not warming up the new converters is on the shop. They should be made to fix it and eat any costs involved. There may be a problem with the downstream oxygen sensor(s). This trouble code sets when the computer sees too much fluctuation in the signal from the downstream oxygen sensor(s). The signal is supposed to be very close to a flat line. What I'd suggest is clear the computer and drive for a few days with the gas tank between 3/4 and 1/4 full. This will allow the computer to run the diagnostics on the catalytic converters. If the trouble code does not come back you should be good to go. If it does return you'll need a shop with a scan tool that can view the signals from the downstream oxygen sensors live while the vehicle is being driven for proper diagnosis. HTH. -Jim

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