  We investigate the uncertainty in surface abundances and yields of asymptotic
giant branch (AGB) stars. We apply three different mass loss laws to a 1.5
solar mass star of metallicity Z=0.008 at the beginning of the thermally
pulsing asymptotic giant branch (TP-AGB) phase. Efficient third dredge-up is
found even at very low envelope mass, contrary to previous simulations with
other evolution codes. We find that the yield of carbon is uncertain by about
15% and for most other light elements the yield is uncertain at the level of
20-80%. For iron group elements the uncertainty varies from around 30% for the
more abundant species to over a factor of two for the less abundant radioactive
species, like iron-60. The post-AGB surface abundances for this mass and
metallicity are much more uncertain due to the dilution of dredged-up material
in differing envelope masses in the later stages of the models. Our results are
compared to known planetary nebula (PN) and post-AGB abundances. We find that
the models are mostly consistent with observations but we are unable to
reproduce observations of some of the isotopes.
