SYMPTOM DIAGNOSIS
| CAUTION | When battery is disconnected, vehicle computer may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION before disconnecting the battery. |
Symptom checks cannot be used properly unless the problem occurs while the vehicle is being tested. To reduce diagnostic time, ensure steps in BASIC DIAGNOSTIC PROCEDURES, SELF-DIAGNOSTICS - EXCEPT 7-SERIES or SELF DIAGNOSTICS - 7-SERIES articles were performed before diagnosing a symptom. Following symptoms are available for diagnosis
- Engine Does Not Start (Will Not Crank)
- Engine Cranks, But Fails To Start
- Engine Backfires
- Loss Of Power
- Detonation (Engine Knock)
- Stalls At Idle
- Engine Runs Rough (Off Idle)
- Rough Or Unstable Idle
- Engine Misfires All Conditions
- Poor Fuel Economy
- Excessive Emissions
PROBLEM DIAGNOSIS
Intermittent fault testing requires duplicating circuit or component failure to identify the problem. These procedures may lead to the computer setting a fault code (on some systems) which may help in diagnosis. If problem vehicle does not produce fault codes, monitor voltage or resistance values using a DVOM while attempting to reproduce conditions causing intermittent fault. A status change on DVOM indicates a fault has been located.
Use a DVOM to pinpoint faults. When monitoring voltage, ensure ignition is in ON position, or engine is running. Ensure ignition is in OFF position or negative battery cable is disconnected when monitoring circuit resistance. Status changes on DVOM during test procedures indicate area of fault.
See also:
• ON-VEHICLE ADJUSTMENTS