confused

I received the code #13 on my 94 chevy truck witch is the o2 sensor so I replaced it but Im still getting code #13 any ideas?

Did you reset the ECU once you replaced the o2?

If you are getting ONLY the code 13 and not codes 44 or 45, there is likely a sensor or wiring problem. The conditions to set a code 13 are:

"Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds. Engine at normal operating temperature and has been running for at least 2 minutes after start with a throttle angle above 5%."

There are a few reasons this could occur. One could be an artificially rich fuel mixture from a shorted/grounded sensor wire or contaminated sensor. Contamination can be from coolant, oil blowby, or fuel additives.

Another reason could be that the exhaust temperature is not reaching a sufficient level to get the zirconia sensor to generate signal. This happens frequently in header installations.

Another reason could be poor electrical connections at the O² sensor connector, ECM, or a poor ground at the sensor.

Another, albeit more obscure possibility, is that the ECM is not developing the 450mV bias signal to the O² sensor, and thus the signal is being read as low (rich). That can be measured by disconnecting the sensor and measuring the voltage at the vehicle (harness) side of the connector while the ignition is on and engine off.