Q: Code P02A9 Cylinder 4 Balance-Fuel Trim at Limit

The code definition specifically indicates the number four fuel injector is leaking. Fuel trims are used to indicate how much the PCM (powertrain control module) needs to adjust the fuel duty cycle to maintain the proper air fuel ratio. Duty cycle is the amount of time the PCM keeps the injector open as measured in milliseconds. Fuel trim at limit indicates the PCM is unable to adjust anymore to maintain the correct air fuel ratio. In this case, the duty cycle of the number four cylinder is most likely close to zero. With this logic the PCM has determined the injector must be leaking.

Your Cadillac has what is known as direct injection. This means the fuel is being sprayed directly into the cylinders as apposed to spraying the fuel into the intake manifold. This can be a potentially bad condition for the motor so the PCM goes into a safe mode which is what it is telling you with indicator that states engine power decrease. Direct injectors must withstand the very harsh environment of the combustion chamber making them susceptible to failure more often.

Most likely you will need a new injector. But with any diagnosis, this should be confirmed first. Depending on the scanner available, this can be confirmed by monitoring fuel pressure while turning on each injector one at a time. If the number four injector is leaking, there will be no change in fuel pressure when the number four injector is turned on. There is a bit more to this procedure, but this is the basic idea. If an appropriate scanner is not available, the number four injector will need to be removed and tested out of the car.

If you should require further assistance with this repair, I recommend the following inspection; check engine light is on. Be sure to send the technician a note informing them of the code so they can come more prepared to for your repair.

Good luck!