P0408 on 2019-2022 Ram 1500 EcoDiesel: EGR Sensor 'B' Circuit High Causes and Fixes

What's Unique About the 2019-2022 Ram 1500

The 3.0L EcoDiesel engine in the 5th Generation Ram 1500 uses a complex dual-circuit EGR system (both high-pressure and low-pressure loops) to meet emissions standards. Code P0408 specifically flags a problem in the low-pressure circuit's 'B' sensor. Manufacturer documents (TSBs) explicitly warn technicians not to mistakenly replace the high-pressure EGR valve for this low-pressure system code, highlighting a common point of confusion and misdiagnosis on this platform. This distinction is critical because the high-pressure system deals with heavy, sooty exhaust pre-filter 🎬 Watch: How to locate and clean your EcoDiesel's EGR diffuser, while the low-pressure system handles cleaner, post-filter exhaust, leading to different failure patterns.

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 (Malfunction Indicator Lamp) is on.
• Rough or unstable idle.
• Reduced fuel economy.
• Possible hesitation or stalling when coming to a stop.
• Failure to pass an emissions test.
• Engine pinging or knocking under load.

Most Likely Causes

1. Faulty Low-Pressure EGR Sensor ('B') 🔴 High Probability The sensor operates in a harsh exhaust environment and can fail internally, causing it to send a continuous high voltage signal. The sensor is often integrated into the low-pressure EGR valve assembly, which is a known maintenance item on EcoDiesel engines.
   How to confirm: Using a scan tool, check the live data for the EGR Sensor 'B' voltage. If it is stuck high (near 5V) regardless of engine RPM, the sensor is likely bad. You can also test the sensor's resistance with a multimeter if specifications are available. 🎬 See this DIY guide on how to test an EGR valve
   Typical fix: Replace the faulty low-pressure EGR sensor or the entire low-pressure EGR valve assembly it is part of.
   Est. part cost: $100-$250
2. Wiring Harness or Connector Issue 🟡 Medium Probability Engine vibrations and heat can cause wiring insulation to chafe or break, leading to a short to a power source. This will cause a 'Circuit High' reading. On Ram trucks, wiring harnesses are known to rub against engine brackets, the firewall, or other components, creating shorts.
   How to confirm: Visually inspect the wiring harness leading to the low-pressure EGR sensor for any signs of damage, melting, or corrosion. Pay close attention to where the harness is secured or makes contact with the engine block or brackets. Perform a continuity test and check for a short to voltage on the signal wire.
   Typical fix: Repair the damaged section of the wiring harness with a new pigtail or by soldering and heat-shrinking the broken wire. Clean or replace the corroded connector and apply dielectric grease.
   Est. part cost: $10-$50
3. Powertrain Control Module (PCM) Software Glitch ⚪ Low Probability Manufacturers occasionally release software updates to correct overly sensitive diagnostic parameters that can falsely trigger trouble codes. TSB S2025000005 Rev. A specifically addresses P0407 and P0408, suggesting a software update may be the prescribed fix from the manufacturer.
   How to confirm: Check with a Ram dealership or use a high-end scan tool like AlfaOBD to see if there are any available PCM software updates for your vehicle's VIN. The TSB for this code confirms that a software-level fix is a possibility.
   Typical fix: Reprogram the PCM with the latest available software. This must be done by a dealer or a qualified shop with the appropriate tools.
   Est. part cost: $0-$200

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is rare, but if the sensor and wiring have been thoroughly tested and are confirmed to be good, the PCM's internal input circuit could be at fault. This should be the last component to be considered for replacement.

Diagnosis Steps

1. Connect an OBD-II scanner to confirm P0408 is the active code and check for any other related codes.
2. Using the scanner's live data function, monitor the voltage of 'EGR Sensor B'. A reading stuck near 5 volts confirms the 'Circuit High' condition.
3. Turn off the engine and locate the low-pressure EGR sensor 'B'. On the 3.0L EcoDiesel, this involves locating the components of the low-pressure EGR system, which routes exhaust gas from after the DPF back to the turbo inlet. The sensor is typically integrated into the LP EGR valve assembly.
4. Carefully inspect the sensor's electrical connector and wiring harness for any signs of damage, melting, chafing, or corrosion. Pay special attention to areas where the harness is routed near hot engine components or sharp brackets.
5. If the wiring looks good, disconnect the sensor. Check the connector terminals for 5V reference voltage and a good ground using a multimeter.
6. If the reference voltage and ground are present, the sensor itself (or the assembly it's part of) is the most likely culprit.
7. If the reference or ground circuits are faulty, trace the wiring back to the PCM to find the open or short. Check for common chafe points along the engine block and near the firewall.
8. If both the sensor and wiring check out, verify with a dealer if a PCM software update is available for your vehicle by referencing TSB S2025000005 Rev. A before considering PCM replacement.

Parts You'll Likely Need

• Low-Pressure EGR Temperature/Position Sensor ('Sensor B') (OEM #68211310AA (Note: This part number is for the 2014-2019 generation, but is often cited for the LP EGR system. Verify by VIN.)) — This sensor is the primary component that triggers the code. Internal failure is a common cause of the 'Circuit High' reading. It is often sold as part of the complete low-pressure EGR valve assembly.
  Trusted brands: Mopar, Bosch
  OEM price range: $150-$250
  Aftermarket price range: $100-$180

Related Codes That Often Appear With This One

• P0407 — This code is for 'EGR Sensor B Circuit Low'. It is the direct counterpart to P0408 and is mentioned in the same TSB, indicating a fault in the same circuit, just with a different failure mode (low voltage instead of high).
• P0403 — This code for 'EGR Control Circuit Open' can appear if the wiring harness issue that causes P0408 also affects other wires in the same bundle, leading to multiple EGR-related electrical faults.

Technical Service Bulletins (TSBs) & Recalls

• S2025000005 Rev. A: Addresses MIL illumination for P0407 or P0408 codes, suggesting a potential software update fix.
• 9004343: Clarifies that P0408 is related to the low-pressure EGR system, not the high-pressure valve, preventing a common misdiagnosis.

Platform-Specific Known Issues

• A Technical Service Bulletin (S2025000005 Rev. A) was issued for Malfunction Indicator Lamp (MIL) Illumination related to codes P0407 (EGR Sensor B Circuit Low) or P0408 (EGR Sensor B Circuit High), indicating a known software or sensor issue.
• A separate TSB (9004343) explicitly warns technicians against replacing the high-pressure EGR valve for a P0408 code, stating that this DTC is for the low-pressure EGR system. This is a common and costly misdiagnosis.
• Owners on forums report that wiring harness chafing is a common issue on Ram trucks in general, which can lead to various electrical faults including P0408.

Mechanic-Grade Diagnostic Values

• EGR Sensor 'B' Signal Voltage (Live Data) — expected: 0.5V - 2.0V at idle, changing with RPM.. Failure: Voltage is stuck high, above 3.0V, and does not change with engine RPM. A reading near 5.0V is a definitive failure.
• EGR Sensor 'B' Ground Circuit — expected: Near 0V and low resistance to a known good chassis ground.. Failure: Voltage present on the ground wire or high resistance (open circuit) to ground.
• EGR Sensor 'B' Reference Voltage — expected: Approximately 5.0V with Key On, Engine Off (KOEO).. Failure: Voltage is significantly lower than 5.0V or is absent, indicating a PCM or wiring issue upstream of the sensor.

Scan Tool Commands That Help

• wiTECH (Dealer Tool) or equivalent: EGR Valve Test / Actuator Test — This bidirectional command attempts to cycle the EGR valve. While monitoring live sensor data, a technician can confirm if the valve is physically responding to commands from the PCM, helping to differentiate a stuck valve from a sensor or wiring issue.
• AlfaOBD: Live Data Graphing (Particulate Filter gas temperature, EGR sensor voltage, etc.) — Allows for real-time monitoring and graphing of sensor inputs. This is extremely useful for confirming a fix by observing sensor behavior during a test drive or a forced stationary regeneration, as demonstrated by owners fixing related emissions faults.
• AlfaOBD / wiTECH: Forced Regeneration — While not a direct fix for P0408, if other EGR or DPF codes are present, running a forced regen and monitoring sensor data with the scan tool can help diagnose the overall health of the emissions system.

Real Owner Repair Stories

• EcoDieselRam.com forum user (2016 Ram 1500 EcoDiesel with 75k miles (engine is Gen 2, but EGR system issues are highly similar to Gen 3)) — Check Engine Light, followed by 'Service Electronic Throttle' warning and limp mode. Found clogged MAP sensor and oily throttle body.
  ❌ Tried (didn't work) Dealer replaced fuel filter for $360, but the light came back on. Cleaning the MAP sensor did not resolve the limp mode.
  ✅ What actually fixed it Owner opted for a complete EGR and DEF system removal (delete) and a Stage 2 tune from a shop called LightSpeed. This resolved all codes and limp mode, and reportedly improved fuel economy from 11.3 to 9.8 L/100km.

"I Checked Everything" — The Actual Cause

• For a 'Circuit High' code like P0408, the equivalent of a 'smoke test clean' scenario is when both the sensor and the wiring harness test perfectly (i.e., the sensor responds correctly off the vehicle, and the wiring shows no shorts or opens). In this situation, the cause is almost certainly a software glitch within the Powertrain Control Module (PCM). TSB S2025000005 Rev. A exists specifically for this scenario, where the dealer is instructed to re-flash the PCM with updated software to correct over-sensitive diagnostic parameters. This should be done before condemning the PCM itself.

OEM Part Supersession History

• N/A → 68492935AA — This is the specific part number for the 'Valve-Low Pressure EGR' for the 2020 Ram 1500 3.0L EcoDiesel, which is the correct assembly to inspect for a P0408 code.
  Heads up: Do not confuse with the High Pressure EGR Valve (68490212AA) or other EGR valves like 5281410AE (superseded by 5281410AG), as TSB #9004343 explicitly warns that P0408 is a low-pressure system fault.