Related Tests, Information and Procedures
What is the most common problem detected by OBD?Fuel Filler cap
If the fuel filler cap is not properly closed after refueling, the OBD system will detect the vapor leak that exists from the cap not being completely tightened.
If you tighten the cap subsequently, the dashboard light should be extinguished within a few days or after deleting the Fault code. This is not an indication of a faulty OBD system. The OBD system has properly diagnosed the problem and accordingly alerted the driver by illuminating the dashboard light.
Please check the fuel filler cap first when the dashboard light comes on to avoid unnecessary diagnostic time. To check the fuel filler cap turn the cap to the right until you hear a click or the cap reaches the full stop. Make sure that the retaining strap is not caught between the filler pipe and the fuel filler cap. If the light should stay on further in depth evaporative leak diagnosis is required.
Misfire Detection
As part of the CARB/OBD II regulations the Engine Control Module must determine if misfire is occurring and also identify the specific cylinder(s). The ECM will determine severity of the misfire event, and whether it is emissions relevant or catalyst damaging. In order to accomplish these tasks the ECM monitors the crankshaft for acceleration losses during firing segments of cylinder specific firing order. If the signal is implausible an erroneous reference mark can be obtained by the ECM which will result in a misfire fault being set.
Possible causes of cylinder misfire faults (actual field findings):
- Vehicle ran low or out of fuel
- Poor fuel quality (ex. water in fuel, customer uses an additive, etc.)
- Low/high fuel pressure
- Ignition coil
- Fouled spark plug(s)
- Restricted/contaminated fuel injector(s)
- Crankshaft position sensor
- Poor combustion due to low compression or high leakage
- Blocked/restricted Catalyst
Engine Misfire Diagnosis
Engine Misfire is the result of inefficient combustion in one or more cylinders. The causes of Engine Misfire are extensive but can be grouped into the following sub-systems. Consider the charts below as an additional diagnostic aid once the DISplus/MODIC is connected, the correct fault symptom has been chosen and the fault memory has been interrogated. Follow the Test Module as displayed by the DISplus/MODIC.
Ignition System
- A secondary ignition oscilloscope display provides vital information about the ignition system's condition.
- Use the following scope patterns as a guideline for ignition system diagnosis. Use the preset measurement function of DISplus
Evaluation of secondary signal amplitude at idle speed.
1. Normal Ignition Voltage Peak: Spark Plug is OK
2. Low Ignition Voltage Peak: Gap too small (defective)
3. High Ignition voltage peak: Gap too large (defective)
Engine Mechanical Systems
Fuel Quality, Delivery, Injection & Evaporative Systems
Implausible ECM Control Function Or Sensor Input Signals
When diagnosing a Misfire fault code, Remember:
"Misfire" is caused by a defect in the internal combustion engine or a defect in the control of the engine operation.
"Misfire" is the result of improper combustion (variation between cylinders) as measured at the crankshaft due to:
- Engine mechanical defects; breakage, wear, leakage or improper tolerances.
- Excessive mixture deviation; air (vacuum leaks), fuel and all the components that deliver air/fuel into the combustion chambers.
- Faulty ignition; primary, secondary including spark plugs.
- Faulty exhaust flow; affecting back pressure.
- Tolerance parameters; ECM programming.
A Misfire fault code(s) is the "symptom" of a faulty input for proper combustion. When diagnosing a misfire, review the charts to assist you in finding the faulty input.