P0421 - Warm Up Catalyst Below Threshold Bank 1
I have a 2001 Galant DE 2.4 liter with about 150,000 miles. The error started recently and a web search has left me a bit confused. Some say it's that catalytic converter, some say it might be the upstream O2 sensor, others say it could be the downstream O2 sensor. I can change the upstream sensor, but I'll have to put it in the shop for the other two. Is it worth changing one or the other, or both, of the O2 sensors? Or should I bite the bullet and have the cat replaced? Alternately, would a couple of heavy doses of B-12 or Seafoam help?