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Volvo MID 128 PSID 90 FMI 4: Meaning, Causes & Fix

EGR Differential Pressure Sensor — signal below normal range

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Diagnostic Reference

Field Details
Code MID 128 PSID 90 FMI 4
Protocol J1587 MID: 128 PSID: 90
Component EGR Differential Pressure Sensor / EGR Valve
Manufacturer Volvo
Engine Series D13
Severity 🟠 CHECK SOON
SAE Reference SAE J1587/J1708 — MID 128, PSID 90

Possible Causes

  1. EGR delta-P sensor tubes clogged with carbon
  2. Sensor diaphragm ruptured
  3. EGR valve stuck closed
  4. Exhaust manifold pressure port blocked
  5. Sensor 5V reference lost

Diagnostic & Repair Procedure

  1. Step 1: Remove EGR differential pressure sensor tubes and clean with carb cleaner and wire
  2. Step 2: Blow out pressure ports on intake and exhaust manifolds
  3. Step 3: Check EGR valve for free movement with Tech Tool actuator test

Frequently Asked Questions

Is this a false alarm or a real problem?

CHECK SOON codes are rarely false alarms. The ECM has detected a parameter outside normal operating range. Sensor malfunctions can sometimes trigger false readings, but the majority of CHECK SOON codes indicate a developing issue that will worsen if left unaddressed. Use a diagnostic scan tool to verify sensor readings before replacing expensive parts.

Can I diagnose this without a scan tool?

Basic visual inspection can be performed without a scan tool — check wiring harnesses for chafing, connectors for corrosion, and related components for physical damage. However, to verify the fault, view live sensor data, and confirm the repair, a heavy-duty diagnostic scan tool (or a compatible OBD-II adapter with J1939/J1708 support) is strongly recommended for accurate diagnosis.

How soon should I schedule service?

Schedule service within the next 1–3 operating days. While the vehicle can continue operating normally, delaying repair increases the risk of the fault escalating to CHECK NEXT STOP severity. Additionally, some CHECK SOON conditions are precursors to emissions system faults that will trigger a mandatory derate if not addressed in a timely manner.

Diesel Repair Cost & Downtime Estimator

Enter estimated labor hours to calculate repair cost and potential fleet downtime losses. Based on national average diesel shop rate of $150/hr and daily revenue loss of $800/day for a parked truck.

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SPN 4375 FMI 7 🟡 CHECK AT NEXT STOP
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SPN 174 FMI 0 🟡 CHECK AT NEXT STOP
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SPN 3031 FMI 9 🟠 CHECK SOON
DEF Quality Sensor
SPN 107 FMI 16 🟠 CHECK SOON
Air Filter Restriction excessive
SPN 84 FMI 0 🟠 CHECK SOON
Vehicle Speed Signal

Associated Symptoms

Check Engine Light Egr Flow Insufficient Rough Idle Turbo Lag Fuel Consumption Increase

References & Further Reading

Data Provenance: This fault code definition is derived from SAE J1587 standards. MID 128 PSID 90 as defined in SAE J1587/J1708. Diagnostic procedures sourced from Volvo OEM technical service documentation. Not for safety-critical decisions. Consult a certified diesel mechanic before performing any repair. See our full disclaimer.