Proper Break-In Procedure for a New Aftermarket Catalytic Converter

1) Before you start: confirm the car won’t kill the new converter

A new converter can be ruined in minutes if the engine is running rich or misfiring.

Quick checks (do these first):

• No active misfire (P0300–P030x) or fuel trim/rich codes

• No coolant consumption (head gasket) or heavy oil burning

• No exhaust leaks upstream of the converter (manifold, flex, gaskets)

• O2 sensors connected and in good condition (especially upstream)

• If you have a scanner: verify STFT/LTFT aren’t extremely positive/negative at idle and cruise

• If the old converter melted: fix the root cause first (misfire, injectors, fuel pressure, etc.)

2) Install correctly (this matters for break-in)

• Use new gaskets/hardware where applicable

• Make sure the converter is not stressed/twisted in the exhaust system

• Ensure correct orientation (front/rear if applicable) and correct O2 sensor locations

• Tighten fasteners evenly; verify no pre-cat leaks

3) First start: controlled warm-up (no revving)

Goal: bring the converter up to operating temp gently.

1. Start the engine and let it idle 3–5 minutes.

2. Do not rev the engine repeatedly.

3. Watch for:

   • Exhaust leaks

   • Burning smells (some is normal initially)

   • Rattles/contact with underbody

If there’s a misfire, strong fuel smell, or flashing CEL: shut it down and fix the issue before continuing.

4) Heat Cycle #1: easy drive (15–20 minutes)

Goal: stabilize temps without overheating.

• Drive 15–20 minutes at light throttle.

• Keep RPM moderate (generally under ~2,500–3,000 RPM).

• Avoid:

  • Wide-open throttle / hard acceleration

  • High-speed sustained climbs

  • Towing or heavy loads

  • “Italian tune-up” style revving

Try to include steady cruise with a few gentle accelerations.

5) Cool-down completely (important)

Goal: allow the substrate and washcoat to “set” through a full heat cycle.

• Park and shut off.

• Let it cool at least 30–60 minutes (longer is fine).

• Don’t spray water on it; just let it cool naturally.

6) Heat Cycle #2: normal commute driving (20–30 minutes)

• Drive 20–30 minutes with normal traffic flow.

• Still avoid:

  • Full-throttle pulls

  • Aggressive downshifts/high RPM

  • Long idling sessions

7) 100-mile caution window (first 1–2 days of driving)

For the first ~100 miles, keep these rules:

• Avoid sustained high RPM/high load (long steep hills at full throttle)

• Avoid towing or heavy cargo if possible

• Avoid extended idling (more than ~10 minutes at a time)

• If you must idle (delivery, work truck), do short intervals and give it a short drive afterward

8) Post break-in check (quick validation)

After the second heat cycle or within the first day:

• Recheck for exhaust leaks

• Verify O2 sensors are tight and wiring is secured away from heat

• If you have a scan tool:

  • Confirm no misfire counts

  • Verify fuel trims are reasonable at idle/cruise

  • Make sure no new catalyst efficiency codes appear

What to avoid at all times (cat killers)

• Driving with a misfire (especially flashing CEL)

• Running rich (fuel smell, black smoke, bad trims)

• Using silicone sealant that’s not O2-safe near the exhaust stream

• Coolant/oil burning not repaired

• Extended high-load driving immediately after install