Upon arrival I went to roof and started removing panels. Removed inducer and manifold to get to heat exchanger. Removed heat exchanger and installed new one. Then installed bracket for blower assembly so it doesn’t wobble. Ran fan only and verified quiet operation. Restarted unit and ran heat. Unit shutting off on high Limit, first stage heat. Found gas pressure at 2.3 inches, rated for 1.6 inches in first stage. Lowered to spec. Ran second stage and found normal gas flow rate. Checked heat rise and amp draw. Unit operating normal at this time.