6 Diagnostic Trouble Codes You Should Never Ignore in Your Diesel Truck

Warning Lights Aren’t Just Suggestions—They’re Your Truck’s SOS

We’ve all seen it—a little light flickering on the dash, signaling some obscure diagnostic trouble code (DTC). Most drivers brush it off, thinking, “It’s probably nothing.” But let us tell you, ignoring these codes can be the fastest route to a roadside breakdown or a five-figure repair bill.

Unlike minor annoyances like a loose fuel cap, certain codes point to critical engine, fuel, exhaust, or transmission system failures. At ALL-IN-ONE Service & Repair, we’ve pulled dozens of trucks off the road due to ignored warning lights that snowballed into full-blown engine meltdowns or catastrophic transmission failures. And trust us—it’s not pretty.

So, if you’re running a diesel truck—whether it's a daily hauler, a fleet workhorse, or a highway long-runner—here are six DTCs you should NEVER ignore, why they happen, and what can go wrong if you do.

P0299 – Turbocharger Underboost Condition

This code essentially means your turbo isn’t doing its job. The ECM (Engine Control Module) expects a certain level of boost based on throttle input and engine load, but something is preventing the system from reaching it.

Common Causes:

• Leaking or damaged intercooler hoses
  Faulty wastegate or actuator
• Boost sensor failure
• Clogged air filter

Why It’s a Big Deal:

Running without proper boost makes your truck a slug on the road. Not only will you notice reduced power and poor throttle response, but you’ll also see higher exhaust gas temperatures, which put your DPF system and turbo components at serious risk.

Ignoring this code can cause:

• Premature turbocharger failure
• Poor fuel economy
• Soot overload in the DPF system
• Limp mode activation

P0087 – Fuel Rail/System Pressure Too Low

The P0087 code means that fuel pressure in the rail is lower than what’s required. With diesel engines relying on high-pressure fuel delivery for performance and combustion efficiency, low pressure can throw everything off balance.

Causes to Look For:

• Clogged or dirty fuel filters
• Weak or failing fuel pump
• Faulty fuel pressure sensor
• Leaking injector return lines

Consequences:

If left unchecked, this can lead to injector misfires, rough idling, hard starts, and eventually injector failure. And if you’re lucky enough to avoid all that, you’re still dealing with power loss and massive downtime while diagnostics drag on.

P2463 – Diesel Particulate Filter (DPF) Restriction

Your truck’s DPF is responsible for trapping soot and carbon. But when it clogs up—and the system can’t regenerate properly—you’ll see this code. It’s arguably the most expensive code on this list if you ignore it too long.

What Causes It:

• Too many short trips with no highway driving
• Failed or skipped regeneration cycles
• Faulty DPF sensors
• Ash buildup beyond cleaning thresholds

Here’s the Kicker:

Left untreated, this can derate the engine, trigger limp mode, or force a costly DPF replacement—often exceeding $3,000. If your DPF gets so plugged that a forced regen can’t save it, you’re in deep.

P0401 – Exhaust Gas Recirculation (EGR) Flow Insufficient

The EGR system recirculates some of your exhaust gas back into the intake to lower combustion temperatures and cut NOx emissions. When that flow becomes blocked, the ECM throws a P0401.

The Likely Culprits:

• Carbon buildup in the valve or cooler
• Stuck EGR valve
• Faulty differential pressure sensor
• Intake or exhaust restrictions

Why It Matters:

Clogged or malfunctioning EGR systems throw off your air/fuel ratio, affect combustion temps, and reduce engine longevity. Plus, it messes with other emissions systems like your SCR and DPF, which creates a domino effect of code-triggering issues.

P0700 – Transmission Control System Malfunction

Unlike the other codes here, P0700 is a general transmission fault indicator. It simply tells the ECM that something's wrong with the Transmission Control Module (TCM) and that more specific codes can be pulled from a deeper scan.

Common Accompanying Codes:

• Shift solenoid faults (P0750, P0755)
• Gear ratio errors (P0730)
• Torque converter clutch (P0741)

Consequences of Ignoring It:

You could experience rough shifts, delayed gear changes, or even failure to shift entirely. Long-term? Say goodbye to your transmission. And guess what—it’s probably the most expensive item on your truck not covered under a drivetrain warranty if you didn’t maintain it properly.

P0113 – Intake Air Temperature Sensor High Input

This one gets shrugged off a lot. After all, who cares what the intake temperature is, right? Wrong.

Your truck’s Intake Air Temperature (IAT) sensor feeds critical data to the ECM to adjust timing, air-fuel mixture, and turbo boost.

Why It Trips:

• Broken or loose IAT sensor connector
• Faulty IAT sensor
• Open circuit in the wiring harness

Problems It Creates:

A bad IAT reading can throw off everything from regen cycles to EGR function and fuel injection timing. It may not kill your engine overnight, but performance, emissions, and fuel economy will all take a hit.

Pro Tips from the Shop: Don’t Just Clear the Code

Let’s put it bluntly: Clearing the code doesn’t fix the problem. It just hides the symptoms for a while—until they rear their ugly head again, often worse than before.

Here’s what we recommend at ALL-IN-ONE Service & Repair:

• Always run full diagnostics using OEM-level tools
• Look at freeze frame data to see when the code triggered
• Check supporting systems, not just the main component
• Log every code and symptom, even if intermittent
• Never ignore repeated codes—they’re persistent for a reason

How Preventive Diagnostics Can Save You Thousands

If you manage a diesel fleet or rely on your truck for a living, the best defense is a preventive maintenance strategy built around early diagnostics.

Get regular checks on:

• DPF differential pressure readings
• Fuel rail pressure logs
• Turbo actuator movement and boost levels
• EGR valve cycling
• Transmission fluid temps and shift patterns

Just because you don’t see smoke or smell burning doesn’t mean your systems are happy.

Conclusion

When it comes to diesel truck diagnostics, certain trouble codes are warning shots—not just minor inconveniences. They’re the canaries in the coal mine, telling you that something deeper, bigger, and much more expensive is just around the bend.

From turbo underboost issues and fuel pressure failures to DPF blockages and transmission red flags, these six codes are the ones you can’t afford to ignore. Waiting too long to act could mean losing not just money—but also valuable time on the road.

At ALL-IN-ONE Service & Repair, we’re built to find these problems before they become disasters. If your check engine light is on, don’t roll the dice. Stop in, scan it, and solve it.

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