a) Increase fuel usage
b) More usage of engine oil, carbon build up will be faster, effecting spark plugs etc.
c) If nothing is done about this, pistons will start to wear out and finally and overhaul will be needed.

Example 2:

When your WCPU (Water Chilled Package Unit) starts producing warm air it is normally because the condensers are full of scale, rust and mud! However most of the time “they” tell you to change the compressors which cost thousands of Dollars!!!

This is what happens :

Due to the sedimentation, heat transfer does not take place. Therefore condensation of R22 gas does not occur. R22 goes back to the receiver tank in mainly gas form, which will eventually damage the compressor and “they” will be right - you have to change the compressor for $$$$$$$ and keep changing it frequently because the condensers are still not cleaned! Regular cleaning and maintaining the condensers will not only keep the air cool but prolong the lifespan of the compressors and save you $.