99 Mitsubishi Galant, Check Engine light came on?

I was having a problem with a 98 galant, same problem (P0170 Fuel Trim Malfunction (Bank 1) & P0171 - System Too Lean (Bank 1)). It wound up needing a complete tune up (ie plugs, plug wires, distributor cap, rotor, air filter, fuel filter and pcv valve). Reset the light and been out since.

Check engine lights can activate for a number of reasons many of which are very minor. Anytime the engine control module senses some anomaly it triggers a code into the system and may or may not activate the light. The first thing you should always do is pull over as quickly as is safely possible, open the hood, and check each and every fluid level. While you've got the hood open look for steam or smoke and note if you smell anything that indicates a very hot engine. Look at the belts at the front of the engine to make sure they are in place and not shredded. Once you are positive that the engine has a good oil level, the coolant is at a proper level, and the transmission fluid is indicated on the dip-stick then you can start the car and take it to a good garage or the dealer. They will electronically read the code and let you know exactly what triggered the light to come on. On some vehicles the check engine light comes on simply to remind you that it is time for an oil change. One more important thing. NEVER check the radiator fluid of a hot engine by opening the radiator cap. You can safely check the coolant level by making sure the fluid is between the low and high level lines on the transparent overflow tank.

DTC P0170: Bank 1 Fuel Trim Malfunction

Probable Causes:

- VAF failed

- Injector No. 1, 3 or 5 failed

- Incorrect fuel pressure

- Air intake system leak

- O2 sensor HO2S failed

- Engine Coolant Temperature (ETC) sensor failed

- Intake Air Temperature (IAT) sensor failed

- Barometric Pressure (BARO) sensor failed

- Exhaust system leak

- Incorrect fuel used

- Powertrain Control Module (PCM) failed

There are too many possiblities to guess here. You will need to take it to a shop and have them go through these until a fault is found. There are diagnostic procedures to check all of these listed that only a service technician will know how to do.

your air fuel ratio is off, bank 1 is on cylinder # 1 side if its a 6 cylinder, if it's a 4 cylinder then it's just 1 bank only. your o2 senser sent your computer this code, your probably running too rich, due to one of these posibilities: mass air flow senser could be malfuntioning, you could have a very dirty air filter, or a vacuum leak on the engine some where, a cylinder misfire can cause this. some times just doing a basic tune up can correct this problem.

I had a similar fault code in my VW, turned out to be a loose vacuum hose. The code roughly means that cylinder one has an issue with the air-to-fuel mixture. It might be wise to google the fault code to see if anyone else has had a similar issue.

Have your ignition switch checked out.