P0408 on 2011-2018 Ram 1500: EGR Sensor Circuit High Causes and Fixes

What's Unique About the 2011-2018 Ram 1500

For the 2011-2018 Ram 1500, this code can appear on both the 5.7L HEMI V8 and the 3.0L EcoDiesel V6 engines, which have different EGR system designs. On the 5.7L HEMI, the EGR valve is a common failure point. For the 3.0L EcoDiesel, EGR system issues are notoriously common and can be more complex, sometimes involving the EGR cooler in addition to the valve itself. There was a safety recall (VB1) for the EGR cooler on 2014-2019 models with the 3.0L engine. A Technical Service Bulletin (TSB S2025000005) was also issued for some 2013 models that could resolve this code with a PCM software update.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Rough or unstable idle
• Reduced fuel economy
• Engine hesitation or stumbling
• Engine pinging or knocking under load
• Occasional stalling when coming to a stop
• Failed emissions test

Most Likely Causes

1. Faulty EGR Valve Assembly 🔴 High Probability → Shop Exhaust Gas Recirculation (EGR) Valve The EGR valve is subjected to intense heat and carbon buildup, causing the internal position sensor to fail or the valve pintle to stick. The sensor is usually integrated into the valve, requiring replacement of the entire unit. This is a very common failure on the 5.7L HEMI.
   How to confirm: Use a scan tool to monitor the EGR sensor voltage. If the voltage is stuck high (often near 5 volts) and does not change when the engine RPM changes, the EGR valve/sensor assembly is likely bad.
   Typical fix: Replace the EGR valve and the corresponding gasket. 🎬 Watch this step-by-step walkthrough for replacing the EGR valve.
   Est. part cost: $150-$300
2. Wiring Harness or Connector Issue 🟡 Medium Probability The wiring harness to the EGR valve is routed in the engine bay where it is exposed to high heat and vibration. This can lead to chafed insulation, broken wires, or corrosion in the connector pins. A short to a power source or a loss of the ground circuit can cause a 'Circuit High' reading. Some GM TSBs for similar truck platforms note specific chafe points against engine brackets, the upper control arm, and the shock tower, which could be analogous issues on Rams.
   How to confirm: Visually inspect the wiring harness and connector for any signs of damage. With the key on, engine off, unplug the EGR connector and check for 5V reference voltage and a good ground at the connector pins using a multimeter. 🎬 Watch: A DIY guide on how to test an EGR valve. A 'Circuit High' code can be caused by a loss of the ground connection.
   Typical fix: Repair the damaged section of the wiring harness or clean/replace the connector. Applying dielectric grease to the connector can help prevent future corrosion.
   Est. part cost: $10-$50
3. Carbon Buildup ⚪ Low Probability All EGR systems handle soot-filled exhaust gas, which can lead to carbon deposits. While more likely to cause a P0401 (Insufficient Flow) code, heavy buildup could potentially cause the valve to stick in a position that results in a high sensor reading.
   How to confirm: Remove the EGR valve and inspect the valve and the passages in the intake manifold for heavy carbon deposits.
   Typical fix: Clean the EGR valve and passages. If cleaning doesn't resolve the issue, the valve may be damaged internally and require replacement.
   Est. part cost: $10-$20 for cleaning supplies

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is very rare. Before condemning the PCM, a software update should be checked for, as per TSBs. All other possibilities, especially wiring and the EGR valve itself, must be exhaustively ruled out first.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm P0408 is the active code. Note any other codes present.
2. Using the scanner's live data function, monitor the EGR Position Sensor voltage. It should typically be between 0.5V and 4.5V and change with engine RPM. A reading stuck high (e.g., near 5V) points to the issue.
3. Turn the ignition off and locate the EGR valve. On the 5.7L HEMI, it's on the front of the passenger side cylinder head.
4. Inspect the electrical connector and wiring harness going to the EGR valve for any signs of melting, chafing, or corrosion. Pay close attention to areas where the harness may rub against engine components.
5. Disconnect the EGR connector. With the key on, engine off, use a multimeter to check for a 5-volt reference signal and a good ground at the connector terminals.
6. If wiring and power/ground are confirmed good, the EGR valve assembly is the most likely culprit.
7. If you have access to a bi-directional scanner, command the EGR valve to open and close while watching the sensor voltage to confirm a lack of response.
8. If replacing the EGR valve, it is also wise to inspect the EGR passages for heavy carbon buildup and clean them if necessary.

Parts You'll Likely Need

• EGR Valve (OEM #Mopar 4593896AB (for 5.7L HEMI, verify by VIN)) — This is the most common cause of P0408, as the position sensor that triggers the code is almost always integrated into the EGR valve assembly.
  Trusted brands: Mopar, Standard Motor Products (SMP), Delphi
  OEM price range: $200-$300
  Aftermarket price range: $150-$250
• EGR Valve Gasket (OEM #Included with most new EGR valves) — The gasket should always be replaced when the EGR valve is removed to prevent exhaust leaks.
  Trusted brands: Mopar, Fel-Pro
  OEM price range: $10-$20
  Aftermarket price range: $5-$15

Related Codes That Often Appear With This One

• P0406 — P0406 is 'EGR Sensor A Circuit High'. On systems with multiple sensors, or if the generic code reader misinterprets the specific Ram code, this can appear. Both point to a high voltage signal from an EGR position sensor.
• P0407 — P0407 is 'EGR Sensor B Circuit Low'. This code is mentioned in the same TSB as P0408 and points to the opposite problem (low voltage), but indicates a fault within the same component's circuit.
• P0401 — P0401 is 'EGR Flow Insufficient'. This can occur if the EGR valve is stuck closed due to carbon buildup or mechanical failure, which might also be related to the sensor circuit fault.

Technical Service Bulletins (TSBs) & Recalls

• S2025000005: Malfunction Indicator Lamp (MIL) Illumination. Diagnostic Trouble Codes P0407 – EGR Sensor B Circuit Low, or P0408 – EGR Sensor B Circuit High Set.
• Safety Recall VB1: Affects 2014-2019 Ram 1500 with 3.0L EcoDiesel engine for EGR Cooler failure.
• TSB 09-009-20: Improperly Installed Exhaust Gas Recirculation (EGR) Valve (for 2019+ models, but notes potential installation issues).

Platform-Specific Known Issues

• A Technical Service Bulletin (TSB #S2025000005) was issued for 2013 models for MIL illumination with P0407 or P0408, indicating this is a known issue recognized by the manufacturer. The fix may involve a PCM software update.
• For 3.0L EcoDiesel engines (2014-2018), P0408 can be an early symptom of EGR cooler failure, which was subject to Safety Recall VB1. 🎬 See how this improved EGR cooler fixes a common EcoDiesel failure. A failing cooler can cause other EGR system faults.
• Another TSB for 2019+ models (09-009-20) mentions improperly installed EGR valves from the factory, which could be relevant to late-model fourth-generation trucks as manufacturing processes overlap.

Mechanic-Grade Diagnostic Values

• EGR Position Sensor 'B' Signal Wire Voltage — expected: 0.5V - 2.0V at idle. Failure: Voltage is stuck high, above 3.0V, and does not change with engine RPM.
• EGR Solenoid Coil Resistance (on some electronic valves) — expected: Approximately 20-25 Ohms between specific pin pairs.. Failure: No reading (open circuit) or significantly lower resistance (short) when tested with a multimeter.
• EGR Connector 5V Reference Voltage — expected: Approximately 5.0V with key on, engine off.. Failure: Voltage is significantly lower than 5.0V or absent, indicating a PCM or wiring issue.
• EGR Connector Ground Circuit — expected: Near 0V and good continuity to chassis ground.. Failure: Voltage present on the ground wire or high resistance to ground, indicating a bad ground connection which can cause a 'Circuit High' fault.

Scan Tool Commands That Help

• wiTECH or equivalent bidirectional scanner: EGR Valve Test / Actuator Test — This command attempts to cycle the EGR valve open and closed. While monitoring live data for EGR position, a technician can confirm if the valve is physically stuck or if the sensor is failing to report position changes. This is a definitive test to isolate a bad valve from a wiring problem.
• wiTECH: Flash Reprogram the PCM/ECM — Used when a Technical Service Bulletin (like S2025000005 for 2013 models) indicates a software update is available to correct false MIL illumination for P0408. This should be done after confirming the hardware (valve and wiring) is functional.

Wiring & Ground Locations

• Engine-to-Chassis Ground Strap — A braided ground strap is located on the rear of the driver's side cylinder head, connecting the engine block to the chassis/firewall.. A loose, corroded, or broken main engine ground can cause erratic voltage readings and floating grounds for multiple sensors, including the EGR system, potentially leading to a 'Circuit High' code even if the sensor's dedicated ground wire appears intact.
• EGR Valve Connector — Directly on the EGR valve, which for the 5.7L HEMI is located on the front of the passenger side cylinder head.. This is the primary point for testing the 5V reference, signal, and ground wires. Corrosion or backed-out pins in this connector are a common cause of circuit faults.

Real Owner Repair Stories

• DodgeForum.com user (2007 Ram 1500 5.7L HEMI) — Check engine light with multiple EGR-related codes, including P0405 (Circuit Low), which later became the primary code.
  ❌ Tried (didn't work) Replacing the EGR valve., Clearing the codes with a reader (light came back on immediately).
  ✅ What actually fixed it The issue was not fully resolved in the thread, but the definitive next step provided by an expert was to unplug the EGR connector and use a voltmeter to test the voltage at each pin to diagnose a wiring or PCM issue, as the new part did not fix the problem.

OEM Part Supersession History

• 4593896AB → 4593896AD — Part revision by manufacturer. Often for improved durability or minor design changes.
  Heads up: The original part number 4593896AB was also superseded by 4861674AC and 4861674AD on some platforms, indicating a complex part history. It is critical to verify the correct part number by VIN, as multiple designs exist.

Model Year Variations Within This Range

• 2013: A specific PCM software update was released under TSB S2025000005 to address false P0407 or P0408 codes. This indicates a potential calibration issue specific to this model year that may not be present in others.
• 2014-2018 (3.0L EcoDiesel only): The introduction of the 3.0L EcoDiesel brought a completely different EGR system, including a complex and failure-prone EGR cooler. On these engines, P0408 can be a secondary symptom of a failing EGR cooler, which was subject to a major safety recall (VB1).