If you cannot find a stock box, then the CEL can be turned off by adjusting either the MAF scaling tables or the injector latency tables.
What happens is on long drives the fuel trims build up and thy go WAY positive. The max positive trim on these cars is +12.5%. Basically the ECU sees a lean condition and starts dumping 12.5% fuel to correct it. Once it cannot dump any more the CEL comes on.
Most tuners can fix that for you. I know I can