Think Systems

What is a ‘System’ and why is it important to consider the ‘System’ when working with DTC ‘s?

A system is the collective name for all the components that are involved in the control of a function.
Thinking about the ‘system’ when fault finding DTCs is very important as the control module that monitors and sets the DTC can only do so via the electrical sensor. The control module can not monitor a mechanical failure but only the results as monitored by the electrical sensor.

Below is a simple image showing airflow through a fuel injected engine.
Air is ‘sucked’ through the air filter and ‘pumped’ out of the exhaust pipe. The quantity and temperature of the air passing through the Airflow Meter is monitored by the Engine Control Module via the wiring.Inlet_exhaust_gas_flow
It is easy to see how a blocked air filter will reduced air flow through the filter.It is also easy to link the Diagnostic Trouble Code of perhaps, P0101-MAF Range/Performance (Manifold Airflow ) to the fault. The engine control module will have a pre-set value range for airflow at a determined engine rpm. As the airflow will be reduced by the blocked air filter so the airflow value read by the airflow meter will be reduced and falls outside of this pre-set value range held within the engine control module. The module will therefore log a DTC such a MAF (Manifold Airflow) out of range.

Blocked air filter
If the air filter is not the root cause of the problem of reduced air flow through the Airflow meter then the diagnosis becomes more complicated.
When the full inlet system is considered, the additional vacuum take-off for items such as brake master cylinder servo, EGR (Exhaust Gas Recirculation) solenoid, fuel tank venting etc could also be the root cause. A split brake servo diaphragm will allow an air bleed into the inlet manifold, allowing the engine to draw a percentage of it’s induction air via the brake servo pipe; this air will not be registered by the airflow meter.

Vacuum supply for EGR servo etc
So in this simple example, a split brake servo diaphragm has reduced the measure air flow at low engine speeds and resulted in a DTC set for Manifold Airflow out of range. The driver will of cause also notice that the vehicle brakes are less efficient than normal and the brake pedal feel is harder.
Would you connect poor braking efficiency and a hard brake pedal with the DTC P0101-MAF Range/Performance?
It is not an obvious connection at first glance but when the ‘System’ is considered this connection can be made.