As per Chuck33079, you may have voided the warranty on the exhaust but that has nothing to do with the airbags (unless you somehow messed up the seat wiring - see MJB's comment about seat sensor - when installing the HFCs).
I'd try unplugging the connector under the seat, blow it out with some dry compressed air, then plug it back in. You may need to disconnect the battery to keep from tripping another error. It's an easy job and a LOT cheaper than buying a new seat. Worth a try.
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