Symptom-Based Diagnosis

Preliminary Checks
Before using Diagnosis by Symptom, perform the On-Board Diagnostic (OBD) System Check and verify all of the following items:
- The powertrain control module (PCM) and malfunction indicator lamp (MIL) (Service Engine Soon lamp) are operating correctly.
- There are no DTC(s) stored.
- Scan tool data is within normal operating range. Refer to Typical Scan Data Values
- Verify the customer complaint and locate the correct symptom in the table of contents. Perform the procedure included in the symptom chart.

Visual/Physical Check
Several of the symptom procedures call for a careful visual/physical check. This can lead to correcting a problem without further checks and can save valuable time.
This check should include the following items:
- PCM grounds for cleanliness, tightness and proper location.
- Vacuum hoses for splits, kinks, and proper connections, as shown on the "Vehicle Emission Control Information" label. Check thoroughly for any type of leak or restriction.
- Air intake ducts for collapsed or damaged areas.
- Air leaks at throttle body mounting area, mass air flow (MAF) sensor and intake manifold sealing surfaces.
- Ignition wires for cracking, hardness, and carbon tracking.
- Wiring for proper connections, pinches and cuts.

Intermittents
IMP0RTANT: An intermittent problem may or may not turn on the malfunction indicator lamp (MIL) or store a DTC. DO NOT use the Diagnostic Trouble Code (DTC) charts for intermittent problems. The fault must be present to locate the problem.

Most intermittent problems are caused by faulty electrical connections or wiring. Perform a careful visual/physical check for the following conditions:
- Poor mating of the connector halves or a terminal not fully seated in the connector (backed out).
- Improperly formed or damaged terminal.
- All connector terminals in the problem circuit should be carefully checked for proper contact tension.
- Poor terminal-to-wire connection. This requires removing the terminal from the connector body to check.
Road test the vehicle with a J 39200 Digital Multimeter connected to a suspected circuit. An abnormal voltage when the malfunction occurs is a good indication that there is a fault in the circuit being monitored.
Use a scan tool to help detect intermittent conditions. The scan tools have several features that can be used to locate an intermittent condition. Use the following feature to find intermittent faults:
- Using a scan tool's "Freeze Frame" buffer or "Failure Records" buffer can aid in locating an intermittent condition. Review and record the information in the freeze frame or failure record associated with the intermittent DTC being diagnosed. The vehicle can be driven within the conditions that were present when the DTC originally set.
To check for loss of diagnostic code memory, disconnect the MAP sensor and idle the engine until the MIL (Service Engine Soon lamp) comes on. DTC P0107 should be stored and kept in memory when the ignition is turned "OFF." If not, the PCM is faulty. When this test is completed, make sure that you clear the DTC P0107 from memory.
An intermittent MIL (Service Engine Soon lamp) with no stored DTC may be caused by the following:
- Ignition coil shorted to ground and arcing at ignition wires or plugs.
- MIL (Service Engine Soon lamp) wire to PCM shorted to ground.
- Poor PCM grounds. Refer to the PCM wiring diagrams.
Check for improper installation of electrical options such as lights, cellular phones, etc. Route ignition control module (ICM) wiring away from the ignition coils. Check all wires from the PCM to the ignition control module for poor connections.
Check for an open diode across the A/C compressor clutch and check for other open diodes (refer to wiring diagrams in Electrical Diagnosis).
If problem has not been found, refer to PCM Connector Symptom tables.
- Check the "Broadcast Code" of the PCM, and compare it with the latest Isuzu service bulletins and/or Isuzu EEPROM reprogramming equipment to determine if an update to the PCM's reprogrammable memory has been released. To check the "Broadcast Code," connect the scan tool, then look for "ID info," then select "Broadcast Code." This should display a 4 character code, such as "XBYA" (example only). This identifies the contents of the reprogrammable software and calibration contained in the PCM. If the Broadcast code is not the most current available, it is advisable to reprogram the PCM's EEPROM memory, which may either help identify a hard-to-find problem or may fix the problem. The Service Reprogramming System (SPS) will not allow incorrect software programming or incorrect calibration changes. Refer to the latest Techline information on reprogramming or flashing procedure.