1.5 is high but should still work as you are not getting a consistent error from that. You can cheat and temporarily bypass it by connecting the two leads together. Everything was working. Now you see burner light but it goes immediately and then get the slow alternating flash. Anything else observed? If not a loose connection to the FS then the board is suspect. It could be more sensitive than the old board was. Some bend the FS to change position. Make sure grounding connections are clean and tight. You need a good meter to measure the FS microamps but difficult if it is not lit. Try with the FS disconnected to see if same symptoms. Check again that ground to neutral is still close to zero ohms.