What is the effect of Helium-based surroundings (99% Helium) on electrochemical gas sensors? I am currently working on a project measuring gas composition of car exhaust. In order to dilute the gas concentration down to something electrochemical sensors can read, I diluted the sample with pure helium. 

The following is true for these sensors:


  Importance of Oxygen. The reactions at the sensing electrode (anode) for some gases are as follows:
  \begin{align}
\ce{CO + H2O &amp;-&gt; CO2 + 2H+ + 2e-} \\
\ce{H2S + 4H2O &amp;-&gt; H2SO4 + 8H+ + 8e-} \\
\ce{NO + 2H2O &amp;-&gt; HNO3 + 3H+ + 3e-} \\
\ce{H2 &amp;-&gt; 2H+ + 2e-} \\
\ce{2HCN + Au &amp;-&gt; HAu(CN)2 + H+ + e-}
\end{align}
  Simultaneously, the reactions at the counter electrode (cathode) need oxygen molecules to complete the process:
  $$\ce{O2 + 4H+ + 4e- -&gt; 2H2O}$$
  An inadequate supply of oxygen to complete the reaction will shorten the life of the sensors, hence the sensors will not operate properly.


Does this mean that doing what I intend is going to affect their performance and damage them?
Aiming for dilution of the exhaust gases, you need a gas that is inert, i.e. does not react at these conditions with neither the gases present, nor with the metals present in in the catalytic converter (if there is such device installed) and electrodes of the sensors.

Under this restraint, using a nobel gas was a good idea.  Yet instead of helium, that you still may use to spot leaks in the tubings, I would opt for argon, which is much cheaper per litre.  Note that at ambient conditions, its density as gas is higher than the one of helium and of air and has a slightly lesser thermic conductance.