Could really be a lot of things. First off, at 238k miles, I'd guess you are still on the original cat? Next time you get it tested make sure you take it on the highway and get it nice and hot. The cat cools down quickly once the car is parked and reduces its efficiency. From then on, it's mostly fuel and timing tuning.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth