Cummins 6.7 P042E — EGR Stuck Open Diagnosis

Introduction

P042E on a 6.7 Cummins is one of those codes where a lot of shops just throw a new EGR valve at it and call it a day. And sometimes that is the right call. But I have pulled EGR valves off trucks, cleaned them at the parts washer, reinstalled them, and had them work perfectly for another 50,000 miles. The difference between a $50 cleaning job and an $800 parts bill comes down to whether you actually diagnose what is going on.

This article walks through the complete EGR system on the 6.7, what P042E means, how to test it, and how to make the right call on repair. This is one of the most common codes on the platform, so if you work on RAM trucks, you are going to see this code. A lot.

How the Cummins 6.7 EGR System Works

The EGR system on the 6.7 Cummins has three main components that work together to control exhaust gas recirculation:

The EGR Valve: Located between the exhaust manifold and the intake manifold, the EGR valve is an electronically controlled valve that meters how much exhaust gas gets recirculated back into the combustion process. It has a DC motor actuator and a built-in position sensor. The ECM commands a position from 0% (fully closed, no recirculation) to 100% (fully open, maximum recirculation) depending on operating conditions.

The EGR Cooler: Before the recirculated exhaust gas enters the intake manifold, it passes through the EGR cooler. This is a heat exchanger that uses engine coolant to drop the temperature of the exhaust gas. Cooler exhaust gas is denser and more effective at reducing combustion temperatures and NOx formation. The EGR cooler is also a known failure point — cracked coolers leak coolant into the exhaust or intake.

The Intake Throttle Valve: The intake throttle valve works in coordination with the EGR valve. By partially closing the throttle valve, the ECM creates a lower pressure in the intake manifold, which creates a pressure differential that helps pull exhaust gas from the higher-pressure exhaust manifold across through the EGR valve and into the intake. Without the throttle valve creating that differential, EGR flow would be limited.

All three components are ECM-controlled and all three can cause drivability issues when they malfunction. But P042E specifically points to the EGR valve itself not closing when commanded.

What P042E Actually Means

The ECM continuously monitors the EGR valve position sensor. When it commands the valve to close (0% position) and the position sensor reports that the valve is still partially open after the expected response time, P042E sets. The threshold and timing vary by calibration, but typically the valve needs to be stuck more than 10-15% open when commanded closed for the code to set.

This is a rationality check — the ECM is comparing what it asked for versus what it got. The important thing to understand is that this code is based on the position sensor reading. If the position sensor itself is faulty and reporting a wrong position, the code can set even if the valve is actually closing properly. That is why you test — you do not just assume the valve is stuck.

Symptoms of an EGR Stuck Open

When the EGR valve is genuinely stuck open, exhaust gas is being recirculated into the intake when it should not be. That causes several noticeable symptoms:

• Excessive black smoke: especially at idle and light load when the ECM does not want any EGR flow
• Rough or unstable idle: the extra exhaust gas dilutes the intake charge and disrupts combustion
• Loss of power under acceleration: the engine is breathing exhaust gas instead of fresh air
• Elevated exhaust gas temperatures: less efficient combustion means more heat energy going out the exhaust
• Possible EGR cooler stress: if hot exhaust gas is constantly flowing through the cooler, it accelerates thermal fatigue on the cooler

On fleet trucks, the driver might describe it as "the truck feels lazy" or "it smokes a lot more than it used to, especially at stoplights." Those are EGR-stuck-open symptoms all day long.

Diagnostic Approach

Step 1: Scan Tool EGR Position Data

Connect your scan tool and pull up the EGR PIDs. You need:

• EGR commanded position (what the ECM wants)
• EGR actual position (what the position sensor reports)

With the engine idling and warm, the ECM will typically command 0% EGR. Look at the actual position. If it reads 0% and tracks correctly when you command different positions through bi-directional controls, the valve is working and the code may have been intermittent or related to a specific operating condition. If actual shows 15-20% or more when commanded to 0%, the valve is physically not closing.

Step 2: Bi-Directional Test

Using your scan tool, command the EGR valve through its full range — 0%, 25%, 50%, 75%, 100%, and back to 0%. Watch the actual position track the command. What you are looking for:

• Does actual track commanded smoothly? If yes, valve is working — may have been a one-time sticking event from carbon
• Does actual lag behind commanded? The motor is weak or the valve is binding on carbon
• Does actual not change at all? Motor is dead, wiring is open, or the valve is completely seized
• Does actual not return to 0% from higher positions? Carbon is preventing full closure — this is the classic P042E scenario

Step 3: Physical Inspection

If the scan tool confirms the valve is not reaching the commanded position, it is time to pull the valve and look at it. Remove the EGR valve from the engine and inspect:

• Valve pintle and seat: carbon buildup on the pintle and around the seat is what prevents the valve from fully closing
• Valve bore: heavy carbon deposits in the bore can restrict valve movement
• Actuator motor: with the valve off the engine, apply battery voltage directly to the motor terminals and see if the valve moves. If the motor runs but cannot move the pintle, it is weak
• Position sensor: if the motor moves the valve freely but the position sensor reading does not match physical position, the sensor is the issue

Why Carbon Is the Number One Cause

Diesel exhaust contains particulate matter — soot. The EGR valve sits directly in the flow path of that sooty exhaust gas. Every time the valve opens, soot-laden exhaust flows across the valve pintle, through the bore, and past the seat. Over time, carbon deposits build up on every surface the exhaust touches.

The problem gets worse under certain conditions:

• Extended idle time: at idle, combustion is less efficient and soot production is higher. Fleet trucks that idle a lot (utility trucks, delivery vehicles) carbon up their EGR valves much faster than highway trucks.
• Short trips: the engine and exhaust system never get hot enough to burn off light carbon deposits.
• Oil consumption: if the engine is consuming oil — even a small amount past the turbo seals — that oil vapor mixes with soot and creates a tar-like deposit that is much harder to clean than dry soot.
• Cracked EGR cooler: as mentioned above, coolant mixing with soot creates an especially aggressive deposit.

There is no way to completely prevent EGR carbon buildup on a diesel engine. It is a maintenance item. The question is how long it takes and whether cleaning or replacement is the better option when it happens.

Cleaning vs Replacement

When to Clean

If the valve is stuck due to carbon buildup but the actuator motor is strong and the position sensor is accurate after cleaning, reinstall it. Remove the valve, soak it in a quality EGR/carbon cleaner or intake cleaner, and work the pintle back and forth by hand to break up deposits. Use a plastic or brass scraper on the seat — do not gouge the sealing surface with a steel tool.

After cleaning, apply voltage to the motor and verify smooth, full-range movement. Check position sensor output at several positions and verify it tracks accurately. If everything checks out, reinstall, clear codes, and road test. Monitor EGR position data on the road test to confirm proper operation.

When to Replace

Replace the EGR valve when:

• The actuator motor is weak — it can barely move the pintle even after cleaning
• The valve seat or pintle is scored, pitted, or warped — it will never seal properly
• The position sensor is inaccurate or erratic — it does not track smoothly with valve movement
• This is the second or third cleaning in a short mileage interval — the valve is worn out

When you replace the EGR valve, take the opportunity to inspect and clean the EGR cooler and intake throttle valve as well. If one is carboned up, the others are too. Cleaning the valve but leaving a carbon-packed cooler and throttle just means everything gets dirty again faster.

Related Codes: P042F and P0401

P042F — EGR Stuck Closed

P042F is the opposite problem — the ECM commands the EGR valve to open, and the position sensor reports it is staying closed. Same carbon buildup cause in many cases, just in the other direction. Carbon can also prevent the valve from opening if deposits build up around the pintle in the closed position. Diagnosis follows the same approach: scan tool position data, bi-directional test, physical inspection.

P0401 — Insufficient EGR Flow

P0401 is a flow-based code rather than a position-based code. The ECM detects that EGR flow is less than expected even though the valve appears to be in the correct position. This can be caused by a restricted EGR cooler (packed with carbon), a restricted EGR passage in the intake manifold, or an intake throttle valve that is not closing enough to create the proper pressure differential. P0401 with a functioning EGR valve almost always points to a restriction downstream of the valve.