Use this commands and it should work
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track 1 ip sla 1 reachability
delay down 30 up 30
ip sla 1
icmp-echo 8.8.8.8 source-interface <interface?
frequency 6
ip sla schedule 1 life forever start-time now
event manager applet clear_nat_up
event track 1 state up maxrun 40
action 1.0 wait 35
action 2.0 cli command "enable"
action 3.0 cli command "clear ip nat trans for"
action 4.0 syslog msg "WAN Failover, cleared NAT"
event manager applet clear_nat_down
event track 1 state down maxrun 40
action 1.0 wait 35
action 2.0 cli command "enable"
action 3.0 cli command "clear ip nat trans for"
action 4.0 syslog msg "WAN Failover, cleared NAT"
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