Caterpillar · SCR Catalyst Exhaust Differential PressureMedium

Caterpillar SPN 4358 FMI 1: SCR Catalyst DP Below Normal

SPN 4358 FMI 1 (FMI 1) — SCR catalyst differential pressure reading is below normal operating range — FMI 1 indicating a plugged sample port, failed sensor, or cracked pressure reference line

4.8/5· 128 reviews

Quick Answer

SPN 4358 FMI 1 on a Caterpillar engine means the SCR exhaust differential pressure is reading below normal range. Inspect the pressure reference tubes for DEF crystal blockage before replacing the sensor.

Overview: Caterpillar SPN 4358 FMI 1

Caterpillar SPN 4358 FMI 1 signals the SCR catalyst exhaust differential pressure sensor is reading below the normal operating floor on C13 and C15 ACERT aftertreatment systems. Unlike FMI 0 or FMI 16 (above normal, indicating a blocked catalyst), FMI 1 means the reading is implausibly low — suggesting the sensor input is being starved of the inlet pressure signal. At ProfessionalDieselRepair.com, the leading cause of this code is a blocked inlet pressure reference tube clogged with DEF crystals or soot, not a failed sensor. Clear both tubes with compressed air before any sensor is ordered. Use Texa or Jaltest to access the full SCR DP parameter set including raw voltage — Cat ET alone does not always expose the FMI 1 root cause on ACERT aftertreatment.

Repair Snapshot

Time

1 - 3 Hours

Difficulty

Intermediate

Parts Cost

$80 - $250

Dealer Labor

$400+

Tools Required

  • Texa Diagnostic Tool
  • Jaltest CDG Interface
  • Digital Multimeter (DVOM)
  • Compressed Air and Blowgun
  • 6mm Hex (M6 barb fittings)

What SPN 4358 FMI 1 Means

SPN 4358 FMI 1 on a Caterpillar C13 or C15 ACERT engine indicates the SCR catalyst exhaust differential pressure sensor is reporting a value below its expected operating range. The SCR DP sensor monitors the pressure drop across the SCR catalyst brick to infer catalyst condition and exhaust flow. FMI 1 (below normal, most severe) means the ECM is reading an implausibly low differential — below 0.3 kPa under load conditions that should show at least 0.5 to 2 kPa. Texa and Jaltest software can read this sensor live and provide bi-directional tests for the aftertreatment module.

Symptoms Drivers Notice

  • Amber aftertreatment warning lamp illuminated on the Caterpillar instrument panel.
  • SCR catalyst DP live data reads below 0.3 kPa even under moderate engine load.
  • Aftertreatment system may inhibit active regeneration or DEF dosing due to unreliable DP data.
  • Additional SCR efficiency or NOx conversion faults may appear alongside SPN 4358 FMI 1.
  • DEF consumption may increase if the system attempts to compensate for an unknown catalyst state.

Most Likely Root Causes

Blocked SCR inlet pressure reference tube (DEF crystallization or soot)

Common

The high-pressure sample port on the SCR inlet side of the sensor can become plugged with DEF crystal deposits or soot bridges, preventing the inlet pressure from reaching the sensor diaphragm and producing a near-zero DP reading.

Failed SCR DP sensor stuck at minimum output voltage

Common

Internal sensor failure causes the differential pressure output to drop below 0.25 V even with both reference ports open to atmosphere, indicating the diaphragm or reference cell has degraded.

Cracked or disconnected pressure reference tube

Occasional

A cracked reference tube equalizes the pressure on both sides of the sensor diaphragm, resulting in a near-zero differential reading that triggers FMI 1.

Open 5 V reference supply to the DP sensor

Occasional

A fault in the aftertreatment module 5 V sensor supply circuit causes the output to drop below the FMI 1 threshold. Check Pin A reference voltage before condemning the sensor.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR DP sensor without inspecting the reference tubes

Why it happens: The sensor is easy to access and inexpensive, making it a default parts-cannon target. Technicians skip the tube inspection step and replace a good sensor that then produces the same low reading because the inlet tube remains plugged.

Do this instead: Disconnect both reference tubes at the sensor and blow them clear with compressed air before making any diagnosis. A blocked inlet tube is the leading cause of FMI 1 on this sensor.

Replacing the SCR catalyst based on the DP reading alone

Why it happens: A technician unfamiliar with FMI directionality may assume a low DP fault means the catalyst is degraded and has low restriction.

Do this instead: A degraded catalyst shows low SCR efficiency faults (SPN 4364), not necessarily SPN 4358 FMI 1. Verify the DP sensor circuit before performing catalyst diagnosis.

Overlooking a cracked reference tube hidden behind the insulation blanket

Why it happens: The flexible pressure tubes are routed under the aftertreatment insulation wrap and are not visible without removing the wrap.

Do this instead: Remove the insulation wrap from the SCR canister section and inspect the full tube run for cracks, kinks, or disconnected fittings.

Master Tech Commentary

Mechanic's Notes

The most frequently missed step on SPN 4358 FMI 1 is the pressure reference tube inspection. Both tubes must be free of DEF crystal buildup (white, chalky deposits) and soot bridges. If the inlet tube is partially blocked, the sensor sees near-zero differential even when exhaust is flowing — this is a clean read that perfectly mimics sensor failure. Heat the tubes briefly with a heat gun to soften any DEF crystals before attempting to clear them with compressed air. The full SCR DP sensor service procedure for C13 and C15 ACERT engines is available at ProfessionalDieselRepair.com. Use Jaltest or Texa for all Cat aftertreatment work since Cat ET does not cover all aftertreatment parameters on older ACERT models.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect a Texa Truck diagnostic interface to the Caterpillar 9-pin ATA connector. Navigate to the aftertreatment system live data group and confirm SPN 4358 FMI 1 is active. Note the current DP reading in kPa. At idle, a correctly functioning sensor should read 0.5 to 2.0 kPa; at rated load, 3 to 8 kPa. A reading below 0.2 kPa at any load condition is the FMI 1 trigger.
  2. Step 2: Locate the two 6 mm diameter flexible pressure reference tubes connecting the SCR inlet and outlet to the differential pressure sensor body mounted on the aftertreatment frame. Disconnect both tubes from the sensor barbed fittings. Blow compressed air (below 30 psi) through each tube to check for blockage. DEF crystallization or soot accumulation can plug the inlet-side tube and cause an artificially low DP reading.
  3. Step 3: With both tubes disconnected and the key on (engine off), back-probe the sensor signal pin to sensor ground. With no pressure applied to either port, a working sensor should output close to the zero-pressure baseline (approximately 0.5 V, corresponding to 0 kPa differential). A reading below 0.25 V with both ports open to atmosphere indicates the sensor is failed low.
  4. Step 4: Measure the sensor 5 V reference supply: with the sensor connected and key on, Pin A (reference) to Pin C (ground) should read 4.95 to 5.05 V. An absent 5 V reference traces to the aftertreatment control module supply circuit. Use Jaltest CDG software to read the ACM sensor supply voltage parameter for cross-check.
  5. Step 5: If tubes are clear and the 5 V reference is present but the signal is below 0.25 V, replace the SCR differential pressure sensor. After replacement, clear faults with Texa and run the vehicle under load. Confirm the DP reading increases proportionally with engine load.

Live Data & Freeze-Frame Tips

Using Texa or Jaltest connected to the Cat ATA port, navigate to the SCR aftertreatment group and monitor SPN 4358 (SCR catalyst DP) alongside engine load and exhaust mass flow. Under light load (1,200 RPM no load), expect 0.5 to 1.5 kPa. Under rated load, expect 3 to 8 kPa. A flat reading that does not respond to RPM or load changes (stays below 0.2 kPa) is the signature of a blocked inlet tube or failed sensor. Compare against SPN 4360 (SCR inlet temp) and SPN 4363 (SCR outlet temp) to build a complete picture of SCR system health.

Safety Warnings

  • Allow the SCR canister and associated piping to cool completely before handling pressure reference tubes — surface temperatures during active regen exceed 500 degrees C.
  • Use appropriate respiratory protection when working near the SCR catalyst and DEF system — DEF vapor and ammonia off-gassing are respiratory irritants.
  • Do not exceed 30 psi when clearing pressure reference tubes with compressed air — excessive pressure can blow out the tube or damage the sensor diaphragm.
  • Wear chemical-resistant gloves when handling DEF-contaminated components.

When to Limp In vs Tow

SPN 4358 FMI 1 alone does not require an immediate tow. However, if the vehicle has been accumulating NOx faults alongside this code for more than a few hours of operation, address the issue promptly to prevent emissions non-compliance and potential derate escalation.

Prevention Tips for Fleets & Owner-Operators

  • Clean the SCR DP sensor reference tubes at every DPF service interval to remove DEF crystal accumulation.
  • Inspect the flexible pressure reference tube insulation for heat damage at every 150,000-mile PM.
  • Use only high-quality, ISO 22241-compliant DEF to minimize crystallization in the dosing and sensing circuits.
  • Verify SCR DP sensor 5 V reference at each PM to catch sensor supply circuit degradation early.

Related Fault Codes

SPN 4358 FMI 2

FMI 2 on the same sensor indicates data erratic or intermittent, which may accompany FMI 1 if the blockage is partial or the sensor diaphragm is intermittently functional.

SPN 4364 FMI 18

SPN 4364 SCR NOx conversion efficiency low — often follows SPN 4358 FMI 1 if the aftertreatment system cannot calculate accurate catalyst performance without valid DP data.

SPN 4360 FMI 1

SPN 4360 SCR inlet temperature FMI 1 may co-occur if the aftertreatment harness bundle has common damage affecting multiple sensor circuits.

Frequently Asked Questions

What does Caterpillar SPN 4358 FMI 1 mean?

SPN 4358 FMI 1 means the SCR catalyst exhaust differential pressure sensor is reporting a value below its normal operating range — almost always caused by a blocked pressure reference tube or failed sensor, not an actual catalyst problem.

What should the SCR exhaust DP read on a Cat C15?

At idle to light load, the SCR DP should read 0.5 to 2.0 kPa. Under rated load, expect 3 to 8 kPa. A reading below 0.2 kPa that does not respond to load changes indicates a blocked inlet tube or sensor failure.

Can I drive a Cat-powered truck with SPN 4358 FMI 1 active?

Yes, briefly. The code alone typically does not cause immediate derate. However, without valid DP data the system cannot monitor SCR catalyst condition, so regen and NOx efficiency monitoring may be impaired.

How do I clear a blocked SCR DP reference tube?

Disconnect both pressure reference tubes at the sensor barbed fittings. Apply no more than 30 psi of compressed air through the inlet-side tube to blow out DEF crystal deposits or soot. A clog will be evident from resistance and then release when cleared.

Which diagnostic tools are needed for Cat SPN 4358?

Texa Truck diagnostic software with a Cat ATA adapter reads the SCR DP live data and provides aftertreatment bi-directional tests. Jaltest CDG also supports Caterpillar C13 and C15 ACERT aftertreatment diagnosis including SCR sensor parameters.

How much does fixing SPN 4358 FMI 1 cost on a Caterpillar?

If the fault is a blocked tube, repair is essentially free — time to clear the blockage. If the sensor is failed, replacement costs $80 to $250. Dealer labor for diagnosis and sensor replacement runs $300 to $500.