1992 Ford Taurus check engine light

Hello,
My friends 1992 Ford Taurus GL (3.0 engine) has a check engine light on. We have taken it to 3 diagnostic centers and they say there are NO codes in the computer to extract. We need to smog this car in CA, which cannot be done with this light on. I have NO idea what might be causing this light to be on as the car runs perfectly.
I have replaced the gas cap, main computer and throttle position sensor, but that is it.

If anyone has an idea what might cause the light, or what to trouble-shoot, please advise!

Thanks

Is the light on quite bright, or just a dim glow? When the key is first turned to Run, is the CEL brighter for the first few seconds, then flicker or dim?

Maybe there is a grounding issue causing the light to appear to be on. Do the scan tools get a code 111 indicating System Pass, or just no communication? Have you tried access the lamp fault codes (http://autorepair.about.com/cs/troubleshooting/l/bl_dtcs_18a.htm)?

-Rod

Hello, thx for the reply. I did not see it sooner as it went into a spam folder. Anyway, it looks to be solved. A mechanic tore the harness apart and found a bad connection. The light was solid and bright when the problem was there. Using a scanner, I pulled two fauld codes, one was for the TPS and the other related to the ignition. After that reading, we could not replicate the codes and all we would get was the 111 pass code. Then when at the mechanic, the codes reappreared and that is when this mechanic thought to open the harness and found the problem. Thanks all for the reply and help.
Regards,