I know that if you put a 96 ecu chipped with JWT, it will run fine in a 98 max. My question is what codes will show up from the check engine light when the max is running on the 96 ecu?

 

rear o2 sensor code at the least.

 

would these codes affect any performance? the reason I ask is I know that there are certain codes that the ecu throws that would put the engine in a "safe mode" where the computer is trying to prevent any damage to the motor until the ecu is reset.

 

No codes CEL will be triggerd with a 96 ECU in a 98 that will cause a performance issue.