Why is my engine management light on?
To help your BMW engine run as efficiently and smoothly as possible, and to help mechanics diagnose a fault with your engine when it has a issue, your BMW is fitted with several sensors which monitor and report back to the engine ECU, when one of the sensors picks up a fault, whether it be a false reading, no reading or a signal it doesn't understand, the engine ECU will usually display a engine management light on the instrument cluster, depending on the severity of the fault, the engine ECU may revert to a limp home mode setting, enabling you to carry on driving your car home, but with reduced power.
When the engine light is showing on the instrument cluster, the engine ECU will store a fault code in its memory, which can be read using suitable diagnostic equipment, which connects up to a diagnostic socket on the vehicle. In some cases, if a fault is just a one off, after a set period of time and engine starts without the fault reoccurring, the fault code may be deleted and the engine light extinguishes.
If the your engine light is staying on, or you notice there is a lack of power, or the engine isn't running a smooth as usual, then you should get the engine ECU read for any fault codes(trouble codes)
What's a Diagnostic Trouble Code (DTC)?
Diagnostic trouble codes or fault codes are codes that are stored by the engine ECU diagnostic system(basically a small computer), for example, when an issue is detected with a sensor that is outside a pre-defined range, as stored by the ECU. Fault codes can either be displayed as a number or a letter followed by a number.
6. Air Flow Sensor
P0171. System Too Lean (Bank 1)
Fault codes help identify a problem area and any stored should help the technician in locating where a fault might be on the car. The codes should be used along with the vehicle's service manual to determine which systems, circuits or components should be tested to diagnose the fault. When diagnosing a fault, reliance solely on the fault code to tell you exactly what is wrong could see you replacing parts which are not faulty, a thorough diagnosis of the system should be carried out first before condemning a part/sensor.
Breakdown of a diagnostic trouble code.
Diagnostic trouble codes are alphanumeric codes that are used to identify a problem that is present on any of the systems that are monitored by the ECU.
EOBD and OBD II diagnostic codes are composed of five characters; one letter followed by 4 digits, example, P0171 System Too Lean (Bank 1)
The first character is a letter. It identifies the "main system" where the fault originated from. This will be one of the following systems:
B = Body
C = Chassis
P = Powertrain
U = Network (UART)
The second character is a numeric digit. This identifies the type of code.
Codes that begin with P0 are Generic codes, where as codes that begin with P1 are manufacturer specific.
Generic(normally P0xxx), the definition for the code is defined in the EOBD / OBDII standard and will be the same for all manufacturers.
Manufacturer-specific(normally P1xxx), Where manufacturers feel that a code is not available within the generic list, they can add their own codes. The definitions for these are set by the manufacturer.
P0xxx - Generic
P1xxx - Manufacturer-specific
P2xxx - Generic
P30xx-P33xx - Manufacturer-specific
P34xx-P39xx - Generic
B0xxx - Generic
B1xxx - Manufacturer-specific
B2xxx - Manufacturer-specific
B3xxx - Generic
C0xxx - Generic
C1xxx - Manufacturer-specific
C2xxx - Manufacturer-specific
C3xxx - Generic
Network Communication codes.
U0xxx - Generic
U1xxx - Manufacturer-specific
U2xxx - Manufacturer-specific
U3xxx - Generic
The third character defines the specific system or sub-system within the car where the problem is located:
1. Fuel and Air Metering
2. Fuel and Air Metering (injector circuit malfunction only)
3. Ignition System or Misfire
4. Auxiliary Emission Control System
5. Vehicle Speed Control and Idle Control System
6. Computer Output Circuits
The fourth and fifth characters identify the section of the system causing the fault. There is no detailed list of the components indicated by these digits, as each system or sub-system will have many different components.