If it is electronic (not cable driven) and both the speedometer and odometer are not working I would suspect the VSS (Vehicle Speed Sensor) is faulty. The VSS can be found on the tail end of the transmission. If only the speedometer doesn't work I would suspect the dash cluster has to be replaced with a new or good used one. Really need to know what year S10 Blazer you have.
fix vapor lock on 1998 chevy s10 2.2 liter 4 cylinder
Ain't no fixin it, you have to replace it with a new one.
what does it do
will a 245x75x15 fix a 2001 chevy blazer 2dr
there are many things that could potentially be wrong with your case. can u engage your 4wheel?
Need to know if the blower is working or not and if the air coming out is hot, warm or cold.
u have to take the hole thing apart to get to a small screw to take the switch out
check to see if your transfer case has to much fluid in it . remove upper plug .
trying to fix simular problem
Just a suggestion, check your trailer lights harness for corrosion. They were taking the dash lights down in my chevy.
P0420 = Catalyst Efficiency Below Threshold (Bank1). Sounds like you probably need to replace the Bank 1 (drivers side) Catalytic converter.
take it to get fix