Drive Cycle
Most OBDII compliant diagnostic monitoring systems run at some time during normal vehicle operation. However, to obtain the most accurate diagnostic information, the vehicle needs to be put in a variety of conditions to satisfy the enable criteria for all monitors to run completely. The importance of enabling criteria being met is facilitated by Trips or Drive Cycles.
Trips
A Trip is required to meet the enabling criteria and consequently, retrieve updated and accurate information from the ECU. A Trip begins when the engine is turned on, the vehicle driven in specific conditions and the engine turned back off.
TRIPS vary with each monitor since each monitor has different ENABLING CRITERIA. Some FAULTS will turn the Malfunctioning Indicator Light (MIL), also known as check engine light, on in one trip while others in two (depending on the specific monitor). This property is defined as ONE-TRIP or TWO-TRIP monitors. As soon as the MIL is requested on, a TRIP COUNTER is started. This keeps track of how many trips are made after the fault is recorded. If the PCM doesn't detect a fault in the next three trips, the MIL is turned off. If the enabling criteria cannot be repeated for whatever reason, then the MIL will remain on. One or more trips make up a drive-cycle.
Drive-Cycle
A drive cycle is a series of trips, usually one or two, that a vehicle is required to undergo so that the powertrain control module (PCM) can trigger the monitoring systems into readiness states allowing for retrieval of diagnostic data.
After a proper repair has been completed and fault codes cleared, a drive-cycle also, allows for the PCM to run all monitoring systems to determine if the previously reported fault(s) no longer exist in comparison with the current retrieved diagnostic information.
The PCM uses a task manager (or 'diagnostic execute') to manage operations of monitors and keep them from affecting vehicle performance. The task manager also makes sure that the monitors runtime is in proper sequential order and prevents potential conflicts between individual monitors. Before running each monitor, the task manager ensures the operating conditions are met, and the required prerequisite monitors (if applicable) have ran and completed. Again, each monitor test can either be passive, active, or intrusive.
The generic OBDII drive cycle
This process is as a generic OBD II Drive Cycle Guide:
Step 1: Ensure there is no check engine light currently on (look at your dashboard)
Step 2: Connect the device and check monitor status and precondition.
Step 3: Ensure fuel tank is between 1/4 and 3/4 full.
Step 4: Start engine at cold temperatures (less than 86 F) and warm up until engine coolant temp is at least 160 F.
Step 5: Speed up to 40-60 mph at 25% throttle (maintain for 5 minutes)
Step 6: Ensure engine coolant temperature reaches and stays above 190 F.
Step 7: Slow down without using the break to 20 mph (or less).
Step 8: Stop vehicle and allow it to idle for 10 seconds.
Step 9: Turn key off, wait 2 minutes to power down PCM.
Step 10: Turn on the car and repeat steps 5 through 9.
There are 3 different ways monitoring systems perform tests:
PASSIVE test strategies
A PASSIVE test is when the PCM monitors sensors for rationality and out of range responses. This is used to detect obvious faults like open or short circuits. For instance, if the throttle position sensor is reading a closed position and the mass airflow sensor is reading a high airflow, then one of these sensors is reading an incorrect value (since a closed throttle should read a little to no airflow).
ACTIVE test strategies
An ACTIVE test is normally in response to a failed passive test. The PCM takes direct control of the component to determine its functionality. It may command a component to do something and check if it responds to the command.
INTRUSIVE test strategies
An INTRUSIVE test is similar to an active test. The PCM takes control of a component but what makes it intrusive is that the test can have an effect on vehicle performance or emissions levels. For instance, the PCM may command a valve open when it’s normally supposed to be closed just to see if it responds to command.