Check Engine Light

I recently purchased a '98 Ranger, 4.0, 4 x 4. Everytime the fuel tank gets below half the check engine light comes on and stays on until I refill the tank. I took it to Auto Zone and they diagnosed a slow O2 sensor which I replaced but it didn't help the problem.
Any ideas?

I recently purchased a '98 Ranger, 4.0, 4 x 4. Everytime the fuel tank gets below half the check engine light comes on and stays on until I refill the tank. I took it to Auto Zone and they diagnosed a slow O2 sensor which I replaced but it didn't help the problem.
Any ideas?

I vaguely remember something like this. Maybe the fuel cap isn't venting right. Next time it comes on, pop the fuel cap and see if it clears up. If so, replace it.

I recently purchased a '98 Ranger, 4.0, 4 x 4. Everytime the fuel tank gets below half the check engine light comes on and stays on until I refill the tank. I took it to Auto Zone and they diagnosed a slow O2 sensor which I replaced but it didn't help the problem.
Any ideas?

This sounds like an EVAP problem, but you say that it gave a code of slow O2 response. Do you remember the code number?

This sounds like an EVAP problem, but you say that it gave a code of slow O2 response. Do you remember the code number?
no, I didn't get the code number. I will next time though. I did replace the sensor and it stll happens.

no, I didn't get the code number. I will next time though. I did replace the sensor and it stll happens.
Did you replace all the O2 sensors... I know on my 97 Ranger, there are 3 sensors. One in each side exhaust and one after the Cat. Converter
Good Luck