exhaust

Flex Pipe

for 2021 Honda Civic 1.5L I4 Turbo · FWD

Difficulty

Moderate

Time

1.5 h

Tools

Steps

This procedure covers the replacement of the flex pipe section in the exhaust system of a 2016-2021 Honda Civic with the 1.5L turbo engine, including disconnection and reconnection of related exhaust components.

Warnings

⚠️Allow exhaust system to cool completely before beginning work. Exhaust components can remain hot enough to cause severe burns for over an hour after engine shutdown.

⚠Exhaust fasteners are prone to corrosion and may break during removal. Apply penetrating oil at least 30 minutes before attempting removal.

⚠The O2 sensor is a precision component. Avoid dropping or striking it, and never allow contaminants to contact the sensor element.

ℹ️This vehicle may have one or two O2 sensors in the flex pipe area depending on configuration. Identify all sensors before beginning disassembly.

Tools required

Floor jack and jack standsEssential

Wheel chocksEssential

19mm socket and ratchetEssential

14mm socket and ratchetEssential

17mm wrenchEssential

Torque wrenchEssential

Penetrating oil

Wire brush

O2 sensor socket (22mm)Essential

Anti-seize compoundEssential

Exhaust gasket scraper

Pry bar

Parts

Flex pipe assembly × 1 — Use OEM specification
Exhaust flange gasket (turbo to flex pipe) × 1 — Use OEM specification
Exhaust flange gasket (flex pipe to catalytic converter) × 1 — Use OEM specification
Flange bolts and nuts × 6 — M10 exhaust hardware
Exhaust hanger isolators (if deteriorated) × 2 — Use OEM specification

Preparation

Park vehicle on level ground and apply parking brake
Place wheel chocks behind rear wheels
Allow exhaust system to cool completely (minimum 2 hours after engine operation)
Raise front of vehicle using floor jack and support securely on jack stands at front subframe jacking points
Spray all exhaust flange bolts, clamp nuts, and hanger bolts with penetrating oil and allow to soak for 30 minutes
Locate the flex pipe section between the turbocharger downpipe and the catalytic converter
Identify O2 sensor locations on or near the flex pipe assembly

Procedure

1
Disconnect oxygen sensor(s)
Trace the O2 sensor wiring harness from the sensor to the electrical connector. Disconnect the connector by releasing the locking tab. Using a 22mm O2 sensor socket, carefully unthread the O2 sensor(s) from the exhaust system. Turn counterclockwise while supporting the sensor to prevent wire damage. Set sensor(s) aside in a clean location where the tip will not be damaged.
⚠Do not allow the O2 sensor tip to contact any surface or contaminants. Even minor contamination can cause sensor failure.
Torque spec
O2 Sensor45 Nm (33 lb-ft)
2
Support exhaust system
Position a jack or support stand under the catalytic converter to support the weight of the downstream exhaust system. This prevents stress on remaining hangers and makes removal easier. Adjust height so components are supported but not under tension.
3
Remove upstream flange connection
Locate the flange connection between the turbocharger downpipe and the flex pipe inlet. Using a 14mm socket, remove the two or three flange bolts securing this joint. If nuts spin, hold them with a 14mm wrench while loosening bolts. Remove and discard old gasket. Carefully separate the joint using a pry bar if necessary, taking care not to damage flange surfaces.
Torque spec
Flange Bolts45 Nm (33 lb-ft)
4
Remove downstream flange connection
Locate the flange connection between the flex pipe outlet and the catalytic converter or mid-pipe. Using a 14mm socket, remove the two or three flange bolts at this connection. Support the flex pipe as you remove the final bolt to prevent it from dropping. Remove and discard old gasket.
Torque spec
Flange Bolts45 Nm (33 lb-ft)
5
Disconnect exhaust hangers
Locate the rubber exhaust hanger isolators supporting the flex pipe assembly. Using a pry bar or by hand, carefully work the metal hanger loops out of the rubber isolators. The flex pipe assembly should now be free to remove. Inspect rubber isolators for cracks or deterioration and replace if necessary.
6
Remove flex pipe assembly
Carefully maneuver the old flex pipe assembly down and out from between the vehicle frame and engine components. Note the routing and orientation for installation of the new part. Inspect the turbocharger downpipe and catalytic converter flange surfaces for damage or warping.
7
Clean flange surfaces
Using a wire brush and gasket scraper, thoroughly clean both flange surfaces on the turbocharger downpipe and catalytic converter or mid-pipe. Remove all old gasket material, carbon deposits, and corrosion. Flange surfaces must be flat and clean for proper sealing. Inspect for cracks or damage.
8
Prepare new flex pipe
Unpack the new flex pipe assembly and verify it matches the old part in terms of length, flange positions, hanger locations, and O2 sensor bung locations. Install new exhaust flange gaskets on both ends of the new flex pipe, ensuring they are properly seated in the flange grooves.
9
Install new flex pipe assembly
Maneuver the new flex pipe assembly into position, following the same routing as the original. Align the inlet flange with the turbocharger downpipe and the outlet flange with the catalytic converter. Insert the metal hanger loops into the rubber isolators. Do not fully tighten any fasteners yet.
10
Install upstream flange bolts
Install new flange bolts at the turbocharger downpipe connection. Start all bolts by hand to ensure proper alignment. Once all bolts are started, tighten them in a crisscross pattern to the specified torque using a torque wrench. This ensures even compression of the gasket.
Torque spec
Flange Bolts45 Nm (33 lb-ft)
11
Install downstream flange bolts
Install new flange bolts at the catalytic converter connection. Start all bolts by hand, then tighten in a crisscross pattern to the specified torque. Ensure the flex pipe is properly aligned and not under tension before final tightening.
Torque spec
Flange Bolts45 Nm (33 lb-ft)
12
Reinstall oxygen sensor(s)
Apply a thin coat of anti-seize compound to the threads of the O2 sensor(s), being extremely careful not to get any on the sensor tip. Thread the sensor(s) into the bungs by hand first, ensuring proper thread engagement. Using the 22mm O2 sensor socket and torque wrench, tighten to the specified torque. Reconnect the electrical connector(s) and ensure wiring is routed away from hot exhaust components.
⚠Use anti-seize sparingly and only on threads. Keep sensor tip completely clean.
Torque spec
O2 Sensor45 Nm (33 lb-ft)
13
Final inspection and adjustment
Remove the support from under the catalytic converter. Visually inspect all connections to ensure proper alignment and clearance. Check that no exhaust components are contacting the chassis, heat shields, or suspension components. Verify all fasteners are tight and all electrical connections are secured.

Reassembly

Ensure all exhaust hangers are properly seated in their rubber isolators
Verify adequate clearance between exhaust components and vehicle underbody, fuel lines, and brake lines
Confirm O2 sensor wiring is routed away from hot exhaust surfaces and is not under tension
Double-check all flange bolts are torqued to specification

Verification

Lower vehicle from jack stands
Start engine and allow to idle. Listen for exhaust leaks at all connection points
With engine running, carefully inspect all flange connections for signs of exhaust gas leakage (visible smoke or hissing sound)
Rev engine to 2500-3000 RPM and listen for abnormal exhaust noises or rattles
Check for diagnostic trouble codes related to O2 sensors using an OBD-II scanner
After a short test drive, re-inspect all connections while engine is warm (but not hot) to verify no leaks have developed
Monitor for check engine light and verify O2 sensors are functioning properly through several drive cycles