My 98 Suburban (251,000 miles) has been running bad the last couple of weeks and is throwing a CEL, and scanning it shows four codes: P0420 (Bank 1 catalyst effciency), P0430 (bank 2 catalyst efficiency), P0135 (O2 sensor bank 1 sensor 1), and P0305 (cyliner #5 misfire). Occasionally, the CEL will actually flash instead of staying on. I replaced both O2 sensors, pre-cat, as I knew at least one was probably original. Also ran a can of Seafoam thru the gas tank on Sunday and it ran a little better on the highway going to Raleigh. But Monday ran like crap again. I'm thinking two things: Injector clogged or malfunction AND/OR catalytic converters are toast. I found the universal Magnflows for close to $80 each. Any ideas on these symptoms? I've never seen a flashing CEL previously.