This procedure covers removal and replacement of the downpipe (catalytic converter section) on the 2016-2021 Honda Civic 1.5L Turbo, including O2 sensor removal and heat shield reinstallation.

Warnings

Tools required

Parts

• Aftermarket or OEM downpipe × 1 — Use vehicle-specific aftermarket or Honda OEM 18160-5AA-A00
• Exhaust gasket (turbo to downpipe) × 1 — Use OEM specification
• Exhaust gasket (downpipe to midpipe) × 1 — Use OEM specification
• Exhaust flange bolts and nuts × 1 — M10 x 1.25 hardware kit

Preparation

1. Park vehicle on level surface and apply parking brake
2. Place wheel chocks behind rear wheels
3. Allow exhaust system to cool completely if vehicle was recently operated
4. Raise front of vehicle and secure on jack stands at manufacturer jacking points
5. Spray all exhaust flange bolts, O2 sensor threads, and heat shield fasteners with penetrating oil; allow 15-30 minutes to soak
6. Locate negative battery terminal but do not disconnect (O2 sensor can remain connected during removal)

Procedure

1. 1

   Remove downstream oxygen sensor
   Disconnect the downstream O2 sensor electrical connector by pressing the release tab. Using a 22mm oxygen sensor socket, carefully unthread the downstream O2 sensor from the downpipe. Turn counterclockwise and apply steady pressure to avoid breaking the sensor. Set aside for reinstallation or replacement.
2. 2

   Remove heat shield fasteners
   Locate the exhaust heat shield covering the downpipe and catalytic converter area. Remove all heat shield mounting bolts using a 10mm socket. There are typically 3-5 bolts securing the shield to the chassis and downpipe. Carefully remove the heat shield and set aside.
3. 3

   Disconnect exhaust hanger at midpipe
   Locate the rubber exhaust hanger supporting the midpipe just behind the downpipe connection. Using a 12mm socket, remove the hanger bolt. Support the exhaust system to prevent strain on remaining connections. This provides clearance for downpipe removal.
4. 4

   Remove downpipe to midpipe connection
   Locate the 2-bolt flange connecting the downpipe to the midpipe. Using a 14mm socket and wrench, remove both flange bolts and nuts. Keep one wrench on the nut while loosening the bolt to prevent spinning. Once removed, separate the flange connection and remove the gasket. Support the midpipe to prevent it from dropping.
   ⚠If bolts are severely corroded and break, the studs will need to be extracted before reassembly.
5. 5

   Remove turbo to downpipe connection
   Locate the 3-bolt flange connecting the turbocharger outlet to the downpipe. Access is from underneath the vehicle. Using a 14mm socket, remove all three flange bolts. Work slowly and evenly to prevent binding. Once all bolts are removed, carefully pull the downpipe away from the turbo flange and remove the gasket.
   ⚠Support the downpipe weight during removal to prevent damage to turbo housing or studs.
6. 6

   Remove downpipe from vehicle
   With both flanges disconnected, carefully maneuver the downpipe out from underneath the vehicle. It may require rotation and angling to clear subframe components and the chassis. Inspect all mounting points and gasket surfaces for damage.
7. 7

   Clean all flange surfaces
   Using a wire brush and scraper, thoroughly clean the turbo outlet flange and midpipe flange surfaces. Remove all old gasket material, carbon buildup, and corrosion. Clean surfaces ensure proper sealing with new gaskets. Inspect turbo flange studs for damage or excessive corrosion.
8. 8

   Install new downpipe
   Apply a thin coat of anti-seize to the turbo flange studs. Position the new gasket on the turbo outlet flange. Carefully position the new downpipe onto the turbo flange studs, ensuring proper alignment. Thread on the three flange nuts finger-tight but do not fully torque yet to allow for alignment adjustment.
9. 9

   Connect downpipe to midpipe
   Position the new gasket between the downpipe and midpipe flanges. Align the bolt holes and insert the flange bolts and nuts. Thread finger-tight. Ensure the entire exhaust system is properly aligned and has adequate clearance from chassis components, fuel lines, and brake lines before final torquing.
10. 10

    Torque turbo flange connection
    Using a torque wrench, tighten the three turbo-to-downpipe flange bolts in a star pattern to ensure even gasket compression. Torque to specification.
    Torque spec

    Exhaust Flange Bolts45 Nm (33 lb-ft)
11. 11

    Torque midpipe flange connection
    Using a torque wrench, tighten the two downpipe-to-midpipe flange bolts evenly. Torque to specification. Verify the connection is secure and the gasket is properly seated.
    Torque spec

    Exhaust Flange Bolts45 Nm (33 lb-ft)
12. 12

    Reinstall exhaust hanger
    Reattach the rubber exhaust hanger to properly support the midpipe section. Install the hanger bolt and torque to specification using a 12mm socket and torque wrench.
    Torque spec

    Hanger Bolts19 Nm (14 lb-ft)
13. 13

    Install downstream oxygen sensor
    Apply a small amount of anti-seize compound to the O2 sensor threads only, being careful to avoid getting any on the sensor tip. Thread the O2 sensor into the downpipe by hand, then torque to specification using the 22mm oxygen sensor socket. Reconnect the electrical connector until it clicks into place.
    ⚠Never apply anti-seize to the sensor tip as this will contaminate the sensor and cause false readings.
    Torque spec

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

    Reinstall heat shield
    Position the exhaust heat shield over the downpipe and align all mounting holes. Install all heat shield bolts and torque to specification using a 10mm socket and torque wrench. Ensure the shield has proper clearance and is not contacting moving components.
    Torque spec

    Heat Shield Bolts10 Nm (7 lb-ft)

Reassembly

1. Verify all fasteners are torqued to specification and no tools remain under vehicle
2. Check entire exhaust system for proper clearance from chassis, suspension, fuel lines, and brake lines
3. Verify O2 sensor electrical connector is properly seated and secured
4. Lower vehicle from jack stands carefully

Verification

• Start engine and allow to idle. Listen for exhaust leaks at both flange connections (hissing or ticking sounds indicate leak)
• Visually inspect all connections for signs of exhaust gas escaping while engine is running
• Rev engine to 2500-3000 RPM and recheck for leaks under load
• Check for Check Engine Light (CEL) related to O2 sensor or catalytic converter after test drive
• Test drive vehicle and verify normal operation, no abnormal noises, and proper exhaust note
• After first 50-100 miles, recheck all flange bolt torque values as exhaust gaskets may settle

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