Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
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What can fail and cause this issue here is the control switch itself on the dash heater controller.It is a vacuum actuated switch as seen in the picture below with the vacuum manifold attachment.
The other issue is this vacuum manifold attachment warps needing replaced as it allows air to leak past it causing this here.In some cases you may see transmission fluid in this attachment and this is due to the switch on top of the transfer case leaking needing replaced then the vacuum manifold replaced at the control head on the dash and the control head mode switch replaced as well.
These are the main causes for your heater/ac system here that needs checked into further and repaired from there.
Code P0155 indicates a fault with oxygen sensor bank 2 sensor 1.This is the sensor located on the passenger side of the engine in the exhaust manifold or pipe that connects to the exhaust manifold.The oxygen sensor itself failing is most common needing replaced to correct this code issue here.
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