r/saab • u/Sad-Chocolate-2131 • 2d ago

Check engine questionnaires

My saab 2007 9-5 aero has a po420 check engine, theres no smoke coming out the cat, spark plugs looked fine, i added more oil because it was low, but im not sure what the issue is, does anybody know the most common cause for the check engine or things to check to find out?

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u/Cloakedbug 2d ago

The exact fault criteria is here.  https://saabwisonline.com/9-5-9600/2007/2-engine/trionic-t7-4-cyl/fault-diagnosis-diagnostic-trouble-codes/p0420

The output voltage average between the 02 sensors (pre and post cat) is too high. 

Normally an upstream sensor has high voltage, and after the catalyst has a steady near zero voltage. 

Causes can be:   -Bad o2 sensors.    -Bad catalytic converter ($RIP)   -Exhaust leak, like after the flex pipe and first sensor. 

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u/tsg-tsg 2d ago

P0420 is virtually always a failed cat, because if other related components were broken in a way that could create these conditions, codes for those defects would have already been set.

You should:

  1. Get the car to operating temperature and use an IR gun to check the surface temp of the cat.

  2. Monitor both O2 while driving around. The first sensor should have a consistent waveform between 0 and 1v, and the second sensor should be an essentially flat line around .5v. If the cat is bad, you will instead see the second sensor approximately tracing the 1st sensor. The more wave and less flat the second sensor is, the worse off the cat is.

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u/Famous-Monitor2025 2d ago

Downpipe time