Common Commercial Vehicle Faults That Cause Inspection Failures

(and the fault codes behind them)

If your trucks are failing inspections, it’s rarely random. The codes were there. The warnings were there. They just didn’t get handled early enough.
For fleet maintenance leaders and lead techs, inspection failures mean downtime, missed loads, and added pressure on a team that’s already stretched.
Let’s break down the most common faults that fail inspections and the actual codes you’ll see when they show up.

Why Commercial Vehicles Fail Inspection

Most failures trace back to active or stored fault codes tied to:

• Emissions systems
• Brake and safety systems
• Engine performance
• Electrical communication

If a code is active and tied to emissions or safety, it’s not passing. Simple as that.

Emissions System Faults (DPF, SCR, DEF)

Why it fails inspection:

Anything affecting emissions compliance is an automatic fail, especially on diesel units.

Common fault codes you’ll see:

• SPN 3251 FMI 0 – Aftertreatment 1 Diesel Particulate Filter Differential Pressure Too High
• SPN 5246 FMI 0/15 – SCR Operator Inducement (DEF system fault leading to derate)
• SPN 3364 FMI 9 – DEF Quality Poor
• SPN 4094 FMI 31 – SCR NOx Conversion Efficiency Low
• P20EE – SCR NOx Catalyst Efficiency Below Threshold
• P207F – Reductant Quality Performance

What this looks like:

• Frequent or failed regens
• Derate warnings
• Check engine light that won’t stay off

How to diagnose it:

With the Autel 909CV:

• Pull full aftertreatment data including DPF pressure and NOx readings
• Run forced regens
• Confirm root cause before replacing parts

With the MD600CV:

• Quickly identify active emissions codes during pre-checks
• Verify if faults are cleared after repair

Why it matters:
These aren’t “clear and go” codes. If you don’t fix the root issue, the truck comes right back or worse, goes into derate on the road.

ABS and Brake System Faults

Why it fails inspection:
Any active ABS fault is a red flag. No exceptions.

Common fault codes:

• SPN 792 FMI 5 – Wheel Speed Sensor Abnormal Frequency
• SPN 639 FMI 2 – CAN Bus Communication Error (affecting ABS module)
• C0035 / C0040 – Wheel Speed Sensor Circuit Fault (OEM-specific)
• SPN 520216 FMI 31 – ABS Configuration or Signal Error

What this looks like:

• ABS warning light on
• Intermittent braking feedback
• Codes that return immediately after clearing

How to diagnose it:

With the Autel 909CV:

• Monitor live wheel speed data across all sensors
• Identify dropouts or inconsistencies
• Pinpoint wiring vs sensor failures

With the MD600CV:

• Fast ABS code scans during intake or inspection prep

Why it matters:

This is safety. Miss it, and you’re risking more than a failed inspection.

Engine Performance and Aftertreatment Support Codes

Why it fails inspection:

Even if the issue starts in the engine, if it impacts emissions or performance, it can trigger a failure.

Common fault codes:

• SPN 102 FMI 3 – Intake Manifold Pressure Sensor Voltage Above Normal
• SPN 157 FMI 18 – Injector Metering Rail Pressure Low
• P0401 – EGR Flow Insufficient
• P0101 – Mass Air Flow Sensor Performance
• SPN 411 FMI 31 – EGR System Performance Issue

What this looks like:

• Poor fuel economy
• Loss of power
• MIL staying active

How to diagnose it:

The Autel 909CV:

• Compare live sensor data against expected values
• Run functional tests on EGR and fuel systems
• Guide you toward root cause instead of guesswork

The MD600CV:

• Quick confirmation of engine-related DTCs before inspection

Why it matters:

These are the codes that get misdiagnosed the most. Fixing the wrong component wastes time and doesn’t clear the failure.

Electrical and CAN Bus Communication Faults

Why it fails inspection:

If modules can’t communicate, systems can’t verify operation. That’s a fail.

Common fault codes:

• SPN 639 FMI 9 – CAN Communication Error
• U0100 – Lost Communication with ECM
• U0121 – Lost Communication with ABS Control Module
• SPN 1231 FMI 9 – J1939 Network Error

What this looks like:

• Multiple unrelated fault codes
• Intermittent system failures
• “No communication” during scans

How to diagnose it:

With the Autel 909CV:

• Full system scan across all modules
• Identify which module is dropping off the network
• Trace communication faults faster

With the MD600CV:

• Identify communication-related codes during quick scans

Why it matters:

These issues eat time. Without the right tool, you’re chasing wiring problems blind.

Transmission and Drivetrain Faults

Why it fails inspection:

Severe drivetrain faults affecting operation or safety can trigger failure conditions.

Common fault codes:

• SPN 521 FMI 2 – Transmission Requested Range Not Achieved
• SPN 558 FMI 18 – Clutch Slip Detected
• P0730 – Incorrect Gear Ratio
• SPN 191 FMI 9 – Output Speed Sensor Error

What this looks like:

• Harsh or delayed shifting
• Slipping under load
• Warning indicators on dash

How to diagnose it:

The Autel 909CV:

• Monitor gear ratios, pressures, and clutch behavior
• Identify early-stage failures

Why it matters:

Catch it early, and it’s a repair. Miss it, and it’s a replacement.

The Bigger Issue: Fault Codes Are Getting Missed

Inspection failures usually aren’t about one bad component.

They’re about a process that didn’t catch the issue early enough.

• Codes weren’t scanned consistently
• Live data wasn’t reviewed
• Techs didn’t have the right tools or support

That’s where your setup matters.

• Autel 909CV gives full-system diagnostics when things get complex
• Autel MD600CV gives fast, reliable scans for daily checks

Together, they help your team stay ahead of failures instead of reacting to them.

Practical Takeaways

If inspection failures are showing up more often than they should:

• Scan every unit before inspection, not after failure
• Don’t just clear codes, verify the fix with live data
• Use quick-scan tools during intake to flag risks early
• Back your techs with tools that actually show what’s going on

Want to Catch These Before They Cost You?

You don’t need more guesswork. You need visibility and a team that knows how to use it.

Triad helps fleets and shops:

• Equip dealer-level diagnostic platforms
• Train technicians to use them right
• Support your team when diagnostics get tough

Talk to a diagnostics expert and tighten up your inspection process before the next failure hits.