Conditions for Diagnosis to Find a Malfunction

Conditions for diagnosis to find a malfunction
For the control module's self-diagnosis of a component or function to start and run (one or several times) special conditions must be fulfilled. Conditions for diagnosis varies depending on which component or function is to be monitored. When the diagnosis has started and is run, the signal, value or function must go above or below the limit value for the control module to detect a malfunction.

Example of conditions for the control module to start the diagnosis/test:
- Engine speed higher than 500 rpm.
- Ignition position II.
- Speed above 40 km/h.
- 4th gear selected.

Example of limit values for detecting malfunction:
- Direct voltage from outside temperature sensor lower than 0.495 V for a time longer than 1 second.
- After the engine has run for approx. 10 minutes, the control module receives an unchanged signal from the oil temperature sensor even though the signal should have changed according to a set pattern.
- Malfunction shall be active (permanent) for at least 2 driving cycles.

Example




The illustration shows an imagined scenario when a malfunction occurs in a circuit for a signal.
First the signal varies normally during driving. The diagnostic trouble code test starts after some time (5), is then run cyclically in a pre-defined pattern and monitors the signal. Limit values for monitoring to consider the signal as incorrect is a voltage lower than 0.50 V (2) or higher than 4.85 V (1). An open circuit occurs in the circuit and the signal becomes 0 V (4). Monitoring detects this, waits so that the malfunction is confirmed and then stores a diagnostic trouble code (6).




If the diagnostic trouble code test (5) starts after the malfunction (4) occurs, then monitoring will detect the malfunction fist when the test starts. Then monitoring waits so that the malfunction is confirmed and then stores a diagnostic trouble code (6). If you want the control module to detect a malfunction in the vehicle, you have to make sure that the control module's diagnostic trouble code test starts or is started.

Hint: If a malfunction occurs which has not yet been detected by the control module's monitoring, since the diagnostic trouble code test has not started, the vehicle may still exhibit a symptom!

Note! Conditions for diagnosis are described in the diagnostic trouble code information for every diagnostic trouble code.

Most diagnosis (at least non-emission-related diagnosis) are repeated periodically during the driving cycle, and every time they are repeated, a test is run that either results in a malfunction being detected or no malfunction being detected.
To enable the emission-related diagnosis to be completed and the diagnostic trouble code to be confirmed (with MIL-light activated as a result), several driving cycles in a row must be completed with a detected malfunction.
To enable the non-emission-related diagnosis to be completed (at least once), in principle the start conditions must be fulfilled for a certain time (from fractions of a second to a number of seconds).
Certain diagnoses require a completed driving cycle (e.g., ignition on, ignition off, and ignition on again) for the diagnosis to be considered as complete. Other diagnoses requires several other conditions to be fulfilled, e.g., with regards to:
- vehicle speed
- engine coolant temperature (ECT)
- time since start
- different load and engine speed relationships in the same trip
- a certain event (for example, that the component operates).

Hint: To assess if a test/diagnosis for the diagnostic trouble code is started or completed, status identifiers can be used. Turning off the ignition and then turning it on again often results in start of a new driving cycle/operation cycle, which results in update of status identifiers. See also section Driving cycle/Operation cycle below.

When, e.g., the engine control module (ECM) has run all diagnostics, the control module has completed a "trip". An extensive driving schedule in various conditions is required to complete a trip. The engine may also need to be shut off for a certain amount of time and then switched on again to complete a trip.
Emissions related diagnostic functions
Control modules that have and control emission-related components or functions, e.g., Engine control module (ECM) and Transmission control module (TCM), check that emission-related systems work. These systems are checked by running a diagnostic function. The diagnostic function checks the included components and that the function itself works in the system.
Lighting the malfunction indicator lamp (MIL)
In the event of emissions related diagnostic trouble codes (DTCs), a counter counts down to determine when to light the malfunction indicator lamp (MIL). The conditions for lighting the malfunction indicator lamp (MIL) vary depending on which diagnostic trouble code (DTC) is stored.

Note! If a malfunction becomes intermittent or the malfunction disappears, then the MIL-light turns off first after 3 driving cycles in a row are completed with malfunction detection.

Self-erasing of emission-related diagnostic trouble codes
After 40 driving cycles without detected malfunction with start of cool engine, where the engine temperature (coolant temperature) increases more than 5 °C (approx. 9 °F) and the final temperature is above approx. +71 °C (159.8 °F), then the diagnostic trouble code is erased. Diagnoses (diagnostic trouble code tests) for misfire and fuel adaptations require 80 driving cycles. These driving cycles are defined as a "warm-up cycle".