Based on my observations which might not be accurate:-
Sengkang services under HGDEP: Sengkang Sq, Compassvale Rd,
Sengkang East Ave, Punggol Rd, Hougang Ave 10, 8, 7, Defu Ave 1 and
straight to HGDEP. Some drivers take Upp Serangoon Rd and Sengkang
East Dr to report from HGDEP to Sengkang interchange.
Service 86 from Sengkang Interchange return to AMDEP via
Sengkang East Rd, TPE, YCK Road and back to AMDEP.
Service 136 from Punggol Interchange return to AMDEP via Punggol
Way, TPE, YCK Road and back to AMDEP.
Hougang services under HGDEP: Hougang Central, Ave 5, 7, Defu
Ave 1 and back to HGDEP.
Not sure about services 159 and 163, which route drivers usually
take between HGDEP and Toa Payoh interchange.