Just like Zodiac said, the ecu goes into open loop when it detects a fault such as code 16 or 45 (fuel). Open loop means non-sensor feedback, another words, your ecu will refer to a fuel and ignition map that is not optimized for performance, but designed to get you home, or to a service facility to take care of the problem. You can check the codes by connecting a jumper or paperclip to your test connector (it's the green one on the right side under the dash, above the ecu) Then the engine light will flash then pause and flash again, one flash pause, then six flashes would be a code 16 for example. If you modified the fuel requirements of the engine, and the ecu is unable to compensate, the primary O2 sensor will read too much or too little oxygen in the exhaust. The best remedy is to find out if the engine is running rich or lean. A good shop can weld a second bung into your exhaust, and attach a wide-band oxygen sensor to test the oxygen content in the exhaust stream. Then a recommendation can be made about engine management, either piggyback, or standalone, or as you mentioned, chipping the ecu. If the oxygen content in the exhaust is too high, then you need more fuel, you can easily buy larger injectors, up to 310cc can be handled by your stock ecu, anything more will require engine management. This response is oversimplified, but gives you an idea of the problem you are dealing with.