Task Guide

Exhaust O2 Sensor Bung Plug — Decat Note

Skill

1/5

Time

25m

When fitting a decat (catalytic converter delete) pipe to an E46 BMW for closed-circuit track use, any O2 sensor bungs left vacant must be plugged with M18x1.5 threaded bung plugs. An open bung allows hot exhaust gases to escape under pressure and draws cold ambient air back into the exhaust stream on deceleration, corrupting the signal of any remaining active lambda sensors and creating an exhaust leak. This is a track-day reference guide applicable to all E46 chassis variants (316i through M3); it is not a road-legal modification in any EU member state including the Netherlands and Germany.

Why it matters

On a decat exhaust pipe the original catalytic converter — and the O2 sensor ports welded into it — are removed as a unit. The replacement decat pipe may have fewer sensor bungs, or none at all, leaving threaded holes open in the exhaust system. An open M18x1.5 bung in a running exhaust system acts as both an exhaust leak and a cold-air intake: under positive exhaust pressure it sprays hot gas outward (fire/burn risk, cabin CO ingestion if near the cabin), and on overrun it draws cold air inward, causing a false-lean signal that drives the ECU into fault mode. On E46 models the downstream (post-cat) sensor is used for catalyst monitoring; with no cat and an open bung, permanent CEL codes will be stored and the closed-loop fuel strategy will be disrupted. Plugging open bungs costs under €10 per bung and takes minutes — leaving them open risks sensor damage, ECU confusion, and a genuine safety hazard at track speeds.

If you delay

Green — 0 to first track session (0 km on decat)
No immediate mechanical damage, but an open bung will trigger a CEL within the first cold-start cycle. Exhaust gas will be audible as a tick or hiss from the bung area. No internal damage yet.
Amber — 1–3 track sessions (~200–500 km on decat)
Persistent CEL locks ECU into open-loop fuelling on some maps. Exhaust heat cycling causes the open bung threads to oxidise and scale, making future plugging progressively harder. Downstream O2 sensor (if still fitted) may read erratically and store fault codes requiring reset.
Red — 3+ track sessions / road use attempted (500–2,000 km)
Thread corrosion in the open bung can advance to the point where a standard plug will no longer seal — requiring re-tapping or bung replacement (welding). If the vehicle is inadvertently used on public roads in this state it is unroadworthy and uninsured in DE/NL. CO exposure risk in cockpit at low speeds.
Black — Extended neglect / decat pipe returned to road vehicle without O2 sensors refitted
Thread is destroyed by heat-cycling corrosion. Bung requires professional exhaust welding repair. ECU may store permanent adaptations requiring dealer reset. Vehicle fails emissions test (AU in DE, APK in NL) and is legally prohibited from road use.

What you'll encounter

The decat pipe will arrive with bungs either already plugged with temporary transit plugs (soft steel, remove before use) or completely open. Do not assume open bungs are intentional or that the pipe is 'ready to fit' — always count and plug before the first start.
M18x1.5 bung plugs are sometimes listed as 'O2 sensor port plugs' or 'lambda bung caps' and come in two styles: a hex-head solid plug (correct — seals permanently under heat) and a thin sheet-metal cap with a centre hole (incorrect — a dust cover only, not an exhaust seal). Confirm your plugs are solid and rated for exhaust temperatures before fitting.
If you are plugging bungs on a hot or recently-used exhaust, the pipe will retain heat in the bung threads far longer than it appears from the outside. The metal 30 cm either side of the bung can be at 200°C while looking identical to a cold pipe. Always confirm cold with a non-contact thermometer or wait a minimum of 45 minutes after last running.
The most common mistake on this specific task is fitting the decat pipe, starting the engine to 'check it sounds right', and only then noticing the open bungs while the pipe is too hot to work on safely. Plan the plug installation before the first start — lay the plugs out next to the pipe while it is on the floor.

Known engine weaknesses

E46 M54 and M52TU engines use two O2 sensors per bank in the standard configuration (pre-cat and post-cat). When a decat pipe is fitted, both sensor locations may be present on the new pipe, only one, or neither — the exact count depends on which decat pipe is purchased. Always count the bungs on the new pipe against the bungs on the removed cat before starting the job.
E46 exhaust bung threads (M18x1.5) are exposed to direct flame and condensation cycling. On cars that have seen winter road salt (common in NL/DE), the first 2–3 threads of an open bung can corrode within a single season, meaning a plug that would have threaded in freely at installation will seize or cross-thread if left open and attempted later.
The E46 M3 (S54 engine) uses a more complex exhaust manifold and sensor arrangement than the M54/M52TU six-cylinders. The S54 has individual headers and the sensor bung positions differ — confirm bung count and location against the specific decat pipe purchased for the S54 before ordering plugs.
E46 four-cylinder variants (316i N42/N46, 318i N42/N46) have a known tendency for exhaust manifold studs and surrounding hardware to corrode heavily — if the decat pipe itself is being fitted at the same time, budget extra time for stud extraction. The bung plugging step itself is unaffected but is typically done in the same session.

Parts verification

Before starting, confirm the following: (1) Count the number of open bungs on the decat pipe as it arrived — hold it up to the light and look through each threaded hole to confirm it is open. A solid plug will be visible; an open bung will show daylight. (2) Confirm each open bung measures M18x1.5 by threading a known M18x1.5 bolt (e.g. a spare O2 sensor) in by hand — it should engage immediately and smoothly with no force. If it does not engage, the pipe may have a non-standard thread (some budget pipes use M18x1.5 but with damaged threads from factory — reject and return). (3) Check the plugs you ordered: they must be solid (no centre hole), hex-head, made of steel or stainless steel, and rated for exhaust use (the product listing should state 'exhaust' or '600°C+' rated). A common mis-order is purchasing M18x1.5 oil drain plugs — these are the correct thread but are not rated for exhaust heat cycling and may seize permanently into the bung. Exhaust-specific plugs use a tapered or crush-washer sealing face; confirm yours has one.

Tools required

M18x1.5 exhaust bung plug(s)
Solid hex-head steel or stainless, exhaust-rated (600°C+), with copper or aluminium crush washer or tapered seat
Thread in by hand first to confirm smooth engagement before applying any tool. If resistance is felt in the first two turns, back out and inspect — never force.
Ratchet or breaker bar
3/8" or 1/2" drive
Use a 6-point socket matched to the plug's hex size (typically 22mm or 27mm depending on brand). A 12-point socket on a corroded plug risks rounding the hex.
Socket to match bung plug hex
Typically 22mm or 27mm — verify against your specific plug before starting
Confirm socket size on the plug before it is in the pipe. Access is easy on the bench but can be awkward once the pipe is fitted.
Torque wrench
3/8" or 1/2" drive, range 20–50 Nm
Set to 30 Nm for standard steel plugs with crush washer. Do not exceed 40 Nm — over-torquing will seize the plug in the bung permanently due to heat expansion.
Wire brush
M18 thread-cleaning brush or standard steel wire brush
Clean bung threads on the pipe before fitting the plug — any grit or scale will give a false torque reading and may damage threads.
Non-contact infrared thermometer (recommended)
Any, range to at least 200°C
Point at the bung area to confirm the pipe is below 40°C before handling. Exhaust pipe retains heat invisibly.
Nitrile gloves and safety glasses
Standard workshop PPE
Exhaust scale and wire brush debris cause eye injuries. Gloves protect against residual heat and sharp edges on cut exhaust pipes.

Parts required

M18x1.5 exhaust bung plug (solid, exhaust-rated)
- OEM-equivalent stainless — Stainless steel hex plug with copper crush washer. Will not corrode or seize between track days. Best choice for repeated removal/refitting when swapping back to road spec. ~€8–15 per plug from exhaust specialists (NL/DE market: Supersprint, Milltek accessories, or generic exhaust suppliers on Amazon.de).
- Standard steel with washer — Carbon steel plug, adequate for single-season track use. Will corrode if left in over winter without removal. Cheaper at ~€3–8 per plug. Acceptable if budget is a priority and you will remove them at the end of each track season.
- Avoid: sheet-metal cap plugs — These have a centre hole and are dust covers only — not exhaust seals. Do not use. Identifiable by thin stamped construction and visible hole in centre.

Procedure

Confirm the exhaust pipe and vehicle are completely cold before starting. If the engine has run in the last 45 minutes, wait. Use a non-contact thermometer pointed at the bung area to confirm temperature is below 40°C.
WhyExhaust steel retains heat far longer than it appears from the outside. The bung area can be at 150–200°C while looking identical to a cold pipe. Crush washers must be seated on a cold surface to compress correctly and give an accurate torque reading; a hot bung will give a false-soft torque feel as the metal is expanded.
Look forThe bung is the threaded port on the exhaust pipe — it looks like a large hex nut welded flush into the pipe wall, about 25mm across, with a threaded hole in the centre approximately 18mm in diameter.
Stop ifDo not rely on touch to check temperature — exhaust pipe can cause full-thickness burns on contact without appearing hot. Always use a thermometer or a strict timed wait.
Lay out all bung plugs, the crush washers (if separate), your socket, ratchet, and torque wrench on a clean surface next to the pipe. Count one plug per open bung — count the open bungs on the pipe again now.
⚙ Torque spec — refer to TIS
WhyStarting with everything laid out prevents the most common mistake on this job: fitting the pipe, starting the engine, and only then finding an open bung on a pipe that is now exhaust-hot and inaccessible.
Look forOpen bungs on the decat pipe look like short threaded tunnels going through the pipe wall — you can see straight through to the inside bore of the pipe when looking at them directly.
Thread one M18x1.5 plug into each open bung by hand, without any tools, for the first 3–4 full turns.
WhyHand-threading first confirms the threads are undamaged and correctly matched before any mechanical advantage is applied. A cross-threaded fastener tightened with a ratchet will destroy the bung thread — a €10 plug becomes a welding job.
Feels likeCorrect engagement feels smooth and progressive — you can feel each thread crest pass with almost no resistance. Wrong: any gritty, crunching, or tight feeling in the first two turns means misalignment or damaged threads. Back out immediately and inspect.
Stop ifIf the plug will not hand-thread smoothly, remove it and inspect the bung threads with a torch. If you see crushed or deformed thread crests, do not force the plug — the bung requires re-tapping with an M18x1.5 tap before proceeding. Forcing a plug into damaged threads will either seize the plug or strip the bung entirely.

Verification protocol

Check
Exhaust gas leak test at each bung plug — hold hand 5–10 cm from each plug during cold idle
Expected
No puffs of warm air, no rhythmic ticking sound synced to engine RPM, no visible smoke or discolouration around the bung area within the first 3 minutes of idle
If wrong
Shut engine off. Allow to cool 45 minutes. Remove the suspect plug, inspect crush washer for correct seating and deformation, clean threads, refit with fresh crush washer, retorque to 30 Nm. Repeat leak test.
Check
OBD-II scan for lambda/O2-related fault codes after first start with plugs installed (track config) or sensors reinstalled (road config)
Expected
No active fault codes related to O2 sensors (P013x, P014x series). On track config with plugs only, codes may be present if the ECU expects sensors — this is expected and acceptable for track use only. On road config with sensors reinstalled, zero O2-related codes should be present.
If wrong
If O2 codes persist after sensor reinstallation, confirm electrical connectors are fully seated (they click audibly when correctly latched), confirm anti-seize did not contaminate the sensor tip, and confirm correct sensor part number for the specific engine code. If codes persist after two cold-start cycles, replace the O2 sensor.
Check
Visual inspection of bung plug area after first heat cycle (after the exhaust has reached operating temperature and cooled once)
Expected
No discolouration, no carbon tracking (black sooty marks), and no visible exhaust staining around the plug hex or the pipe surface immediately around the bung
If wrong
Carbon tracking or soot around a bung indicates an exhaust gas leak past the plug seal. Remove, inspect, and replace crush washer — do not attempt to re-torque an already heat-cycled crush washer as it will not compress further.

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