Is Your Vehicle Struggling? Top Signs of Exhaust Fluid System Faults You Shouldn’t Ignore

“Exhaust Fluid System Fault” means your DEF/SCR emissions system isn’t operating correctly due to low DEF, poor/contaminated DEF, or component faults (sensors, pump, injector, wiring, or control module). Common symptoms include dashboard warnings, speed limits (limp mode), reduced power, and in some cases a no‑restart countdown. Quick fix: top up with API/ISO 22241‑compliant DEF, check for leaks/contamination, then drive at a steady speed for 10–15 minutes to allow the system to relearn. If the warning remains, scan for OBD‑II codes (e.g., P207F, P20EE, P204F) and schedule professional diagnostics for sensors/pump/injector/SCR.

Introduction

When your dashboard displays “Exhaust Fluid System Fault,” your diesel’s DEF (Diesel Exhaust Fluid) and SCR (Selective Catalytic Reduction) system has detected a problem. This emissions control system converts nitrogen oxides (NOx) into harmless nitrogen and water. If it malfunctions, your vehicle may enter reduced‑power mode, limit maximum speed, and in some cases begin a countdown to prevent restart. This guide explains the most common causes, symptoms, quick fixes, diagnostic steps, prevention tips, and when to visit KB Tire & Auto Repair in Highland, NY.

Understanding Exhaust Fluid Systems in Modern Vehicles

• Role of DEF in emission control: DEF (32.5% automotive‑grade urea, 67.5% deionized water) is injected into the hot exhaust stream. It forms ammonia (NH₃), which reacts with NOx in the SCR catalyst, reducing up to ~90% of NOx emissions.
• Key components: DEF tank (with level sensor and often a heater), DEF pump, injector, temperature/quality sensors, SCR catalyst, control module (ECU), and wiring/connectors.
• Operating conditions: DEF freezes around 12°F/−11°C (systems typically include heaters). DEF consumption varies by load and driving; refills often range from 2,500–10,000 miles depending on vehicle and conditions.

Symptoms: What You’ll Notice

• Dashboard warnings: “Exhaust Fluid System Fault,” “Service DEF System,” or “Reductant System Fault.”
• Performance issues:
  • Speed limited (commonly ~5–20 mph depending on the vehicle).
  • Limp mode with reduced power and limited RPM.
  • No‑restart countdown: the system may display remaining starts if the fault persists.
• Hard starting or no restart after shut‑off.
• Check Engine light may accompany emissions warnings.

Common Causes

• Low DEF level (empty/near empty or misread levels).
• Poor/contaminated DEF (non‑compliant, expired, diluted, or foreign contamination).
• Sensor failures (level, temperature, or quality sensors).
• Pump/injector issues (weak pump, clogged injector, insufficient flow).
• Electrical faults (loose connectors, wiring damage, blown fuses, control module errors).
• Frozen DEF (system inoperative until thawed in cold weather).
• SCR efficiency problems (aging/fouled catalyst, exhaust restrictions).

Quick DIY Checklist (5 Steps)

1. Check the DEF level on the gauge or at the tank; top up with API‑certified, ISO 22241‑compliant DEF if low.
2. Use fresh, sealed DEF from a reputable source; avoid home‑mixed or unknown brands. Do not use water or additives.
3. Inspect for leaks at the tank, lines, and connectors; wipe any spills to prevent surface corrosion.
4. After refilling, drive steadily for 10–15 minutes to allow the system to relearn; perform a key‑off/key‑on power cycle and recheck the warning.
5. If the warning persists, likely culprits are sensor/pump/injector/electrical faults. Proceed to diagnostics.

Diagnosing: Scan for Trouble Codes (DTCs)

Use an OBD‑II scanner to read codes. Common DTCs include:

• P207F — Reductant quality performance (poor DEF quality or quality sensor issues).
• P20EE — SCR NOx catalyst efficiency below threshold (catalyst or sensor concerns).
• P204F — Reductant system performance (pump/injector/sensor performance problems).
• P205B — Reductant tank level sensor circuit (level sensor fault).
• P20BA — Reductant heater control circuit (DEF heater issue).
• P203B/P203C — Reductant level/pressure related issues.

Notes: Some codes clear only after a complete drive cycle post‑repair or require a scanner reset. If you don’t have a scanner, schedule professional diagnostics to pinpoint the component.

What to Do Next (If–Then Guidance)

• If DEF level is low or quality is suspect:
  • Drain/flush contaminated DEF if necessary.
  • Refill with fresh API/ISO‑compliant DEF; drive steadily 10–15 minutes; recheck the warning.
• If warnings remain or the vehicle is power‑limited:
  • Book diagnostics for level/temperature/quality sensors, pump output test, injector flow test, wiring continuity and connector integrity, and SCR efficiency/backpressure checks.
• If temperatures are very cold:
  • Verify DEF heater operation, park in a warmer garage, and allow thawing. Avoid heavy loads until the system reaches operating temperature.

Preventing Exhaust Fluid System Failures

• Always use API‑certified, ISO 22241‑compliant DEF from reputable sources.
• Store DEF between ~41–77°F (5–25°C), out of direct sunlight; keep containers sealed; respect shelf life.
• Fill properly to avoid air bubbles and misreads; wipe up spills.
• Inspect hoses/connectors for leaks and corrosion; follow manufacturer service intervals.
• Don’t ignore warnings; early intervention prevents limp mode and no‑restart scenarios.

FAQs

Why does my car say “Exhaust Fluid System Fault”?

The DEF/SCR system detected issues with DEF level/quality or component faults (sensors, pump, injector, electrical). The ECU limits performance to control emissions and protect the system.

Can I drive with an Exhaust Fluid System Fault?

You may be able to drive briefly, but expect reduced power and speed limits. A no‑restart countdown may begin. Address it promptly: top up compliant DEF, check for leaks/quality; if the warning remains, scan codes and repair.

How do I reset the fault after refilling DEF?

Drive steadily for 10–15 minutes to allow relearning, then power cycle. If the warning persists, codes may need clearing with a scanner or the underlying issue hasn’t been resolved (sensor/pump/injector/electrical).

What DEF should I use?

Use API‑certified, ISO 22241‑compliant DEF. Avoid expired, contaminated, or unsealed fluid and unknown brands.

Does cold weather cause this fault?

DEF freezes around 12°F/−11°C. Systems include heaters, but extreme cold can trigger temporary faults. Warm storage and verifying the DEF heater reduce cold‑related issues.

Local Expertise and Why Choose KB Tire & Auto Repair

• In Highland, NY, we diagnose and repair DEF/SCR systems for a wide range of diesel vehicles daily. Example: 2018 Ram 2500 — P207F resolved with DEF flush and quality sensor replacement, same‑day turnaround.
• Our certified technicians use professional OBD‑II tools, perform injector flow and pump output tests, and verify SCR efficiency to ensure lasting repairs.
• Call (845) 691-2262 to schedule a DEF/SCR diagnostic. Initial assessment typically takes 60–90 minutes. We provide transparent estimates and follow OEM‑level procedures.

Conclusion

An “Exhaust Fluid System Fault” isn’t always severe, but prompt action prevents limp mode and no‑restart scenarios. Start with a compliant DEF top‑up and a 10–15‑minute steady drive; if the warning persists, scan for DTCs and inspect sensors, pump, injector, wiring, and SCR. KB Tire & Auto Repair in Highland, NY is ready to provide accurate diagnostics and safe, reliable repairs.

About The Author

Liam Kohn

I’m Liam Kohn. I started KB Tire & Auto Repair because I know what it’s like to need your car running right, but not have a fortune to spend on it. Been wrenching for over a decade – tires, brakes, keeping those old engines humming. Whatever it takes to get you back on the road. I’m not about fancy sales pitches, just straightforward advice and honest work. If you’re tired of getting jerked around at the big dealerships, let’s talk.

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