That means your Engine has a fault; unfortunately the only way to find out what is to put it on a Diagnostics setup. It ought to have logged the fault.
Mine was a faulty No. 4 injector but that was accompanied by the engine kangarooing; if its all smooth then it must be either non critical or instantly switching to another input. Most of the sensors have an alternative of some sort but if you have the fault light on then you risk masking a more serious problem.
