MIXTURE SOLENOID

The check engine light does not tell you to change any parts. It merely records data that it sees. The technician then has to determine what caused it. What was the exact code number recorded?

You have a number of different codes there that may or may not be related. Code 23 is telling you there is a problem with the control solenoid circuit. That may or may not be caused by the solenoid itself. It could just as easily be in the wiring or the ECM. Changing the solenoid requires disassembly of the carburetor and that is not something a novice should attempt. It's basically rebuilding the carb and that is no simple task. All the additional codes could indicate an ECM problem or various other possibilities. You may want to let a professional dig into this.

12 No reference pulses to ECM.
13 Oxygen sensor circuit.
14 Short in coolant sensor circuit.
15 Open coolant sensor circuit.
21 Throttle position sensor circuit.
23 Open or grounded mixture control (M/C) solenoid circuit
24 Vehicle speed sensor circuit.
32 Barometric pressure sensor circuit.
34 Manifold absolute pressure or vacuum sensor circuit.
35 Idle speed control circuit shorted.
41 No distributor reference pulses at specified engine vacuum.
42 Electronic spark timing bypass circuit grounded.
44 Lean Oxygen sensor indication.
44 & 55 (At same time) Faulty oxygen sensor circuit.
45 Rich system indication.
51 Calibration unit (PROM) faulty or incorrectly installed.
52 Faulty ECM.
53 Faulty ECM.
54 Shorted MC solenoid circuit and(or faulty ECM.
55 Grounded V ref (term. 21), faulty oxygen sensor or ECM.

That doesn't mean anything.