This procedure covers removal and replacement of the downpipe (catalytic converter assembly) on the 2018-2024 Honda Accord 1.5L Turbo, connecting the turbocharger to the mid-pipe exhaust section.

Warnings

Tools required

Parts

• Downpipe/Catalytic Converter Assembly × 1 — Use OEM specification or aftermarket equivalent
• Turbo-to-downpipe gasket × 1 — 18115-5AA-A00 or equivalent
• Downpipe-to-midpipe gasket × 1 — 18229-5BA-A00 or equivalent
• Exhaust flange nuts (if corroded) × 3 — M10x1.25 copper nuts
• Heat shield hardware (if damaged) × 1 — Use OEM specification

Preparation

1. Park vehicle on level ground and allow exhaust system to cool completely (minimum 2 hours after last operation)
2. Disconnect negative battery terminal to clear engine codes and reset readiness monitors
3. Raise vehicle using jack and support securely on jack stands at manufacturer lift points
4. Spray all exhaust flange bolts, O2 sensor, and hanger bolts with penetrating oil and allow to soak minimum 15 minutes
5. Remove engine undercover screws and remove plastic undertray for full access to downpipe
6. Locate both oxygen sensors: primary sensor in turbo outlet and secondary sensor in downpipe body

Procedure

1. 1

   Remove primary oxygen sensor
   Disconnect electrical connector for primary (upstream) O2 sensor located near turbocharger outlet. Using 22mm O2 sensor socket, carefully unthread primary sensor from turbo downpipe connection. Sensor threads are likely corroded; apply heat if necessary. Store sensor safely if reusing.
   ⚠O2 sensor wires are fragile. Support sensor weight while removing to avoid wire damage.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
2. 2

   Remove secondary oxygen sensor
   Disconnect electrical connector for secondary (downstream) O2 sensor located in downpipe body approximately 18 inches below turbo. Using O2 sensor socket, remove secondary sensor. Keep sensors identified if reusing (primary vs secondary).
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
3. 3

   Remove heat shield fasteners
   Remove heat shield bolts securing the upper and lower heat shields around the downpipe. There are typically 4-6 bolts depending on configuration. Set heat shields aside. Inspect shields for damage and replace if severely corroded.
   Torque spec

   Heat Shield Bolts10 Nm (7 lb-ft)
4. 4

   Disconnect turbo-to-downpipe connection
   Locate the three flange bolts connecting downpipe to turbocharger outlet (top connection). Using 14mm socket with extension and universal joint, remove three exhaust flange bolts. Support downpipe weight as final bolt is removed. If bolts are seized, apply additional penetrating oil and allow to soak, or carefully apply heat.
   ⚠Seized bolts may break. Have replacement hardware available. Do not force frozen fasteners without penetrating oil treatment.
   Torque spec

   Exhaust Flange Bolts45 Nm (33 lb-ft)
5. 5

   Disconnect downpipe-to-midpipe connection
   Locate the three-bolt flange connection where downpipe connects to midpipe/center exhaust section (lower connection). Remove three 14mm flange bolts. This connection is typically more accessible than upper connection. Separate flanges carefully.
   Torque spec

   Exhaust Flange Bolts45 Nm (33 lb-ft)
6. 6

   Remove exhaust hangers
   Locate rubber exhaust hangers supporting downpipe assembly (typically two hanger points). Use hanger removal tool or carefully pry hangers off mounting studs using large flathead screwdriver. Work hangers off studs without tearing rubber if components are being reused.
   Torque spec

   Hanger Bolts19 Nm (14 lb-ft)
7. 7

   Remove downpipe assembly
   With all connections and hangers disconnected, carefully maneuver downpipe assembly out from underneath vehicle. Downpipe may need to be tilted and rotated to clear subframe and steering components. Remove old gaskets from turbo outlet flange and midpipe connection flange.
   ℹ️Downpipe weighs approximately 15-20 lbs and is awkward to maneuver. Have a helper or use a support stand.
8. 8

   Clean mating surfaces
   Using wire brush, thoroughly clean turbo outlet flange and midpipe flange mating surfaces. Remove all old gasket material, carbon deposits, and corrosion. Flanges must be clean and flat for proper sealing. Inspect flange faces for warping or damage.
9. 9

   Prepare new downpipe
   Inspect new downpipe for shipping damage and verify O2 sensor bung threads are clean. Apply anti-seize compound to O2 sensor threads (threads only, not sensor tips). Install new turbo-to-downpipe gasket onto downpipe upper flange. Install new downpipe-to-midpipe gasket onto lower flange.
10. 10

    Install downpipe assembly
    Carefully maneuver new downpipe into position, aligning upper flange with turbo outlet and lower flange with midpipe. Start all three upper flange bolts finger-tight first, then start all three lower flange bolts finger-tight. Do not fully tighten yet. Reinstall rubber exhaust hangers onto mounting studs.
11. 11

    Torque flange connections
    Apply anti-seize compound to threads of all flange bolts. Using torque wrench, tighten turbo-to-downpipe flange bolts (upper connection) to specification in star pattern. Then torque downpipe-to-midpipe flange bolts (lower connection) to specification in star pattern. Verify gaskets remain properly seated.
    ℹ️Proper torque sequence prevents gasket crushing and ensures even clamping force. Tighten in multiple passes if necessary.
    Torque spec

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

    Install oxygen sensors
    Thread primary O2 sensor into turbo outlet bung by hand until seated, then torque to specification using O2 sensor socket. Thread secondary O2 sensor into downpipe body bung and torque to specification. Reconnect both O2 sensor electrical connectors ensuring secure latching.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
13. 13

    Reinstall heat shields
    Position heat shields around downpipe in original locations. Install heat shield bolts and torque to specification. Ensure heat shields do not contact downpipe directly and have proper clearance to prevent rattling.
    Torque spec

    Heat Shield Bolts10 Nm (7 lb-ft)
14. 14

    Reinstall undercover and lower vehicle
    Reinstall plastic undertray and secure with cover screws torqued to specification. Verify all tools and parts have been removed from under vehicle. Carefully lower vehicle from jack stands.
    Torque spec

    Cover Screws10 Nm (8 lb-ft)

Reassembly

1. Reconnect negative battery terminal and verify connection is secure
2. Double-check all exhaust connections are properly torqued and O2 sensor connectors are latched
3. Verify exhaust hangers are properly seated and heat shields are secured without contact points

Verification

• Start engine and listen for exhaust leaks at both flange connections (ticking or hissing sound indicates leak)
• Check for fault codes using OBD-II scanner; address any O2 sensor circuit codes immediately
• Allow engine to reach operating temperature and verify no unusual odors or smoke from exhaust components
• Perform test drive and verify no exhaust rattles, vibrations, or abnormal sounds under acceleration
• After 50-100 miles of driving, re-check all flange bolt torque as new gaskets may settle
• Monitor for check engine light; O2 sensor readiness may take 50-100 miles of mixed driving to complete

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