Check Engine Light Diagnosis

Diagnose the cause of an illuminated Check Engine Light on a 5.9L Cummins diesel using an OBD-II scanner to retrieve diagnostic trouble codes and perform systematic testing of related components.

Warnings

Tools required

Parts

• Shop towels × 1 — N/A
• Electrical contact cleaner × 1 — CRC 05103 or equivalent
• Dielectric grease × 1 — Use OEM specification

Preparation

1. Park vehicle on level ground and engage parking brake
2. Allow engine to cool if recently operated to prevent burns during underhood work
3. Ensure battery is in good condition with at least 12.4V to prevent diagnostic communication issues
4. Locate OBD-II diagnostic port under driver side dashboard near steering column
5. Have pen and paper ready to record all diagnostic trouble codes and freeze frame data

Procedure

1. 1

   Connect Scanner and Retrieve Codes
   Insert the diagnostic scan tool into the diagnostic port and turn ignition to ON position without starting engine. Note that 1994-1995 (and some early diesel) models predate OBD-II and require a Chrysler DRB-style scan tool or key-cycle code retrieval, while 1998+ models use the OBD-II connector under the dash. Power on scanner and select 'Dodge' then 'Ram 2500' and appropriate year. Navigate to 'Read Codes' or 'Retrieve DTCs'. Record all codes displayed including P-codes (powertrain), along with their descriptions and freeze frame data showing conditions when code was set.
2. 2

   Document Code Status
   Distinguish between active codes (current faults), pending codes (intermittent issues detected once), and stored/historical codes (faults that occurred previously but are not currently present). Note how many drive cycles each code has been present. Common Cummins codes include P0216 (injection timing), P1693 (companion codes present), P0234 (turbo overboost), and grid heater relay codes. Note the 5.9L Cummins uses an intake air grid heater, not glow plugs.
3. 3

   Check for Technical Service Bulletins
   Cross-reference retrieved codes with known Dodge/Cummins Technical Service Bulletins for this model year. Common issues include VP44 injection pump failures (especially with low fuel pressure codes P0216), lift pump failures causing stalling, boost/MAP sensor failures (P0237 turbo boost sensor low, P0238 turbo boost sensor high), and grid heater relay issues. Determine if a TSB applies before proceeding with component testing.
4. 4

   Perform Visual Inspection
   Open hood and inspect all visible wiring harnesses, connectors, and vacuum lines for damage, corrosion, or disconnection. Pay special attention to engine harness near exhaust manifold where heat damage is common, intake air temperature sensor wiring, MAP sensor connector on intake horn, and crankshaft/camshaft position sensor connectors at front and rear of engine block. Look for evidence of rodent damage, oil contamination, or coolant intrusion in connectors.
5. 5

   Inspect Air Intake System
   Release air box clips and remove air filter housing cover. Inspect air filter for excessive restriction or damage. Check all intake tubing from air filter to turbo inlet and from turbo outlet to intake manifold for cracks, loose clamps, or oil accumulation indicating turbo seal failure. Inspect intercooler boots for tears or separation which cause boost leaks and trigger MAP sensor codes.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)
6. 6

   Test Sensor Power and Ground Circuits
   Using digital multimeter set to DC voltage, backprobe suspect sensor connectors without disconnecting them. With ignition ON engine OFF, verify 5-volt reference voltage is present at applicable sensors (MAP, IAT, and the APPS/accelerator pedal position sensor on VP44-equipped 1998.5-2002 trucks). Check sensor ground circuits show less than 0.1V drop to battery negative. For two-wire sensors like coolant and intake air temperature, measure resistance values and compare to specification (typically 2000-3000 ohms at 70°F ambient).
7. 7

   Test Crankshaft and Camshaft Position Sensors
   If codes P0335, P0340, P0385, or P0390 are present (applicable to VP44 electronic-pump 1998.5-2002 models; 1994-1998 P7100 mechanical-pump trucks do not use these sensors), locate the crankshaft position sensor and camshaft position sensor per the factory service manual, as locations vary by year. Disconnect each sensor and measure resistance across sensor terminals, comparing readings to the Cummins factory specification. Inspect sensor mounting bolts for proper torque. Check sensor faces for metal debris accumulation and air gap (should be 0.020-0.040 inch).
8. 8

   Inspect Fuel System Components
   For fuel-related codes, verify fuel supply pressure at VP44 injection pump using pressure gauge installed at Schrader valve on fuel filter housing. Lift pump should provide approximately 14-17 PSI at idle on VP44-equipped trucks; sustained pressure below the factory minimum accelerates VP44 injection pump failure. Verify the exact specification against factory service data. If pressure is low, suspect lift pump failure (very common on these trucks). Check the intake air grid heater relay operation by listening for the relay click when the ignition is turned on in cold conditions. Inspect fuel filter housing for air leaks which cause hard starting and surging.
9. 9

   Test Boost and Exhaust Pressure
   For turbocharger-related codes (P0234, P0237, P0238), verify wastegate actuator moves freely and is not stuck. With engine running, verify MAP sensor reading matches actual boost pressure using separate gauge installed in intake manifold. Check exhaust back pressure before the turbo to identify exhaust restriction from a clogged muffler or crushed exhaust pipe (these years have no catalytic converter). Compare back pressure to the factory specification.
   ⚠Turbocharger and exhaust components will be extremely hot during testing; use proper heat protection
10. 10

    Check PCM Communication
    If scanner shows 'No Communication' or if code P1694 is present, verify PCM/ECM connector security at the module (location varies by year; consult factory service manual for exact mounting position). Check PCM power and ground circuits. Verify fuses are intact for PCM power circuits. Check battery voltage during cranking remains above 10V as low voltage causes communication issues and false codes.
11. 11

    Perform Component-Specific Testing
    Based on retrieved codes, perform focused testing on identified components. Clean all relevant connectors with electrical contact cleaner and apply dielectric grease. If component is confirmed faulty through testing, replacement is required. If component tests within specification but code persists, check wiring harness for opens, shorts, or high resistance between sensor and PCM.
12. 12

    Clear Codes and Test Drive
    After repairs or if all systems test normal, use scanner to clear diagnostic trouble codes from PCM memory. Start engine and allow to reach operating temperature while monitoring scanner live data. Verify all sensor readings are within normal parameters. Perform test drive including varied load conditions, acceleration, and highway speeds to verify fault does not return. Scan for codes again after test drive.

Reassembly

1. Reinstall air filter housing cover and secure all clips properly
2. Ensure all sensor connectors are fully seated and locked with no exposed terminals
3. Verify all wiring harnesses are routed away from hot exhaust components and moving parts
4. Double-check that no tools or parts were left in engine compartment

Verification

• Confirm Check Engine Light is extinguished with ignition ON after code clearing
• Verify scanner shows 'No Codes Present' or only historical non-active codes
• Monitor scanner live data stream shows all sensors reading within normal operating ranges
• Test drive vehicle for at least 15 minutes through varied conditions without Check Engine Light returning
• Perform final code scan after test drive to ensure no pending codes are present

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

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