Consider the reaction : $2CO(g)+O_2(g)\rightleftharpoons 2{CO}_2(g)+Heat$
Under what conditions shift is undeterminable?

Increase in temperature and decrease in volume:
As increase in temp. shifts backward but decrease in volume shifts forward.
Addition of CO and increase in temperature at constant volume:
As addition of CO shifts reaction forward but increase in temp. shifts it backward.
According to-
LE-CHATELIER'S PRINCIPLE : According to this principle. If a system at equilibrium is subjected to a change of concentration, pressure or temperature, the equilibrium is shifted in such a way as to nullify the effect of change. 
(a)Change in concentration : In an equilibrium increasing the concentrations of reactants results in shifting the equilibrium in favour of products while increasing concentrations of the products results in shifting the equilibrium in favour of the reactants. 
(b)Change of pressure : When the pressure on the system is increased, the volume decreases proportionately. The total number of moles per unit volume increases. According to Le-Chatelier's principle, the equilibrium shift in the direction in which there is decrease in number of moles. If there is no change in number of moles of gases in a reaction, a pressure change does not affect the equilibrium. 
(c)Change in temperature :- If the temperature at equilibrium is increased reaction will proceed in the direction in which heat can be used. Thus increase in temperature will favour the forward reaction for endothermic reaction. Similarly, increase in temperature will favour the backward reaction in exothermic reactions.