SCR Catalyst

This procedure involves replacing the SCR (Selective Catalytic Reduction) catalyst on a 1994-2007 Dodge Ram 2500 with the 5.9L Cummins diesel engine.

Warnings

Tools required

Parts

• SCR catalyst assembly × 1 — Use OEM specification
• Exhaust flange gasket (inlet) × 1 — Use OEM specification
• Exhaust flange gasket (outlet) × 1 — Use OEM specification
• O2 sensor (if damaged) × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and engage parking brake
2. Place wheel chocks behind rear wheels
3. Allow exhaust system to cool completely (minimum 2 hours after engine operation)
4. Raise vehicle and support securely on jack stands at frame rails
5. Spray all exhaust flange bolts, hanger bolts, and O2 sensor threads with penetrating oil and allow to soak for 15-30 minutes
6. Disconnect negative battery cable to prevent fault codes during sensor disconnection

Procedure

1. 1

   Disconnect oxygen sensors
   Locate the NOx sensors and any exhaust gas temperature sensors associated with the SCR catalyst assembly. Disconnect electrical connectors by depressing tabs and pulling straight out. Label connectors to ensure correct reconnection. Inspect connectors for corrosion or damage.
2. 2

   Remove heat shields
   Remove any heat shields covering the SCR catalyst assembly by removing the heat shield bolts. Set heat shields aside and inspect mounting points for damage or corrosion.
   Torque spec

   Heat Shield Bolts10 Nm (7 lb-ft)
3. 3

   Support exhaust system
   Position a floor jack with wooden block or exhaust support stand under the exhaust system just forward of the SCR catalyst to support weight during removal. Do not rely solely on exhaust hangers to support components.
4. 4

   Disconnect exhaust hangers
   Remove hanger bolts securing the SCR catalyst assembly to the vehicle chassis. Use penetrating oil if hangers are seized. Note the orientation and position of all hangers for reassembly.
   ⚠Exhaust hangers may be under tension. Control components as hangers are released to prevent sudden movement.
   Torque spec

   Hanger Bolts20 Nm (15 lb-ft)
5. 5

   Remove oxygen sensors from catalyst
   Using the appropriate wrench, carefully remove the NOx and temperature sensors from the SCR catalyst housing. Turn counterclockwise and avoid excessive force which may damage sensor threads or housing. If sensors are seized, apply heat carefully with a heat gun (not torch) and additional penetrating oil.
   ⚠Do not use excessive force on O2 sensors. Damaged threads in catalyst housing cannot be repaired and will require housing replacement.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
6. 6

   Disconnect inlet flange
   Remove exhaust flange bolts connecting the inlet pipe to the SCR catalyst. Support the catalyst assembly with the jack while removing bolts. Remove and discard old gasket material from both flange surfaces.
   ⚠Ensure exhaust system is properly supported before removing final flange bolts to prevent sudden drop.
   Torque spec

   Exhaust Flange Bolt45 Nm (33 lb-ft)
7. 7

   Disconnect outlet flange
   Remove flange bolts connecting the SCR catalyst outlet to the downstream exhaust pipe. Carefully lower the SCR catalyst assembly once all bolts are removed. Remove and discard old gasket from flange surfaces.
8. 8

   Remove SCR catalyst assembly
   With assistance if needed due to weight, carefully lower and remove the SCR catalyst assembly from the vehicle. Handle carefully to avoid damage to internal catalyst substrate. Inspect mounting points and surrounding exhaust components for damage or excessive corrosion.
9. 9

   Prepare flanges and mounting points
   Using a wire brush and cleaning solvent, thoroughly clean all exhaust flange mating surfaces, removing all old gasket material, carbon deposits, and corrosion. Inspect flange surfaces for warping or cracks. Clean all hanger mounting points and bolt threads.
10. 10

    Install new SCR catalyst
    Position new exhaust flange gaskets on inlet and outlet flanges. With assistance, lift new SCR catalyst assembly into position and align with inlet and outlet pipes. Support with jack while aligning bolt holes.
    ⚠Ensure gaskets are properly seated and aligned before installing bolts. Misaligned gaskets will cause exhaust leaks.
11. 11

    Install inlet flange connection
    Apply anti-seize compound to exhaust flange bolt threads. Install inlet flange bolts hand-tight first to ensure proper alignment. Once all bolts are started, tighten in a cross-pattern to specified torque.
    Torque spec

    Exhaust Flange Bolt45 Nm (33 lb-ft)
12. 12

    Install outlet flange connection
    Apply anti-seize compound to flange bolt threads. Install outlet flange bolts hand-tight, then tighten in a cross-pattern to specified torque. Ensure gasket remains properly positioned during tightening.
13. 13

    Install oxygen sensors
    Thread the NOx and temperature sensors into the SCR catalyst housing by hand to avoid cross-threading, then tighten to specified torque with the appropriate wrench. Do not add anti-seize unless the sensor manufacturer specifies it, and keep any compound away from the sensing element.
    ⚠Do not apply anti-seize to sensor tips or allow contamination of sensing elements. This will cause sensor failure and inaccurate readings.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
14. 14

    Secure exhaust hangers
    Align SCR catalyst assembly with hanger mounting points. Install hanger bolts and tighten to specified torque. Verify exhaust system has proper clearance from chassis, fuel lines, brake lines, and suspension components throughout full suspension travel.
    Torque spec

    Hanger Bolts20 Nm (15 lb-ft)
15. 15

    Reinstall heat shields
    Position heat shields over SCR catalyst assembly and install heat shield bolts. Tighten to specified torque. Ensure shields have proper clearance and are not contacting moving components.
    Torque spec

    Heat Shield Bolts10 Nm (7 lb-ft)
16. 16

    Reconnect oxygen sensors
    Connect all oxygen sensor electrical connectors, ensuring they click securely into place. Route wiring away from heat sources and moving components. Secure any wiring clips or ties.

Reassembly

1. Reconnect negative battery cable
2. Lower vehicle from jack stands
3. Double-check all exhaust connections for proper torque
4. Verify all exhaust hangers are secure and properly supporting system weight

Verification

• Start engine and allow to idle. Listen for exhaust leaks at all flange connections
• Inspect all connections visually for signs of exhaust gas escape (soot, staining, visible smoke)
• With engine at operating temperature, check for leaks again as gaskets seat under heat
• Use scan tool to verify no oxygen sensor codes are present and sensors are reading properly
• Verify SCR system is functioning correctly with scan tool monitoring of NOx sensor data and DEF injection (if applicable)
• Test drive vehicle and verify no exhaust leaks, unusual noises, or warning lights appear
• After test drive, perform final inspection of all connections and check torque on accessible fasteners

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