1997 GMC Safari (trouble code 0121)

[Fecklar]

Hello,
I have a 97 Safari cargo van with a trouble code 0121. I looked up the code and put in a new TPS switch and cleared the code. The engine seemed to run fine before and after I put in the switch. A day later the engine light came on again with the same code. I used an AC Delco part. The kilometers on the van is 752,000 Km. The engine is original. I know this may seem like a lot of Km's but I have a fleet of these and G vans and if it is well maintained, it's not a problem. Any suggestions would be appreciated.
Regards
Steve

BTW - its a 4.3 Litre engine.