O2 Sensor Spacer Install on a 2005 Chevy Astro

After chasing P0420 through fuel trims, exhaust inspection, and downstream O2 sensor age, we decided the cheapest reversible step was an O2 sensor spacer.

What it is

A small threaded fitting — M18×1.5 on both ends — that installs between the exhaust bung and the downstream O2 sensor. Some contain a mini-catalyst; some are just a small internal restriction. Both accomplish the same thing: the downstream sensor sees a slightly muted version of what's leaving the cat, which is what a healthy exhaust *should* look like.

Tools

• 22mm O2 sensor socket (or open-end wrench if you're brave)
• Anti-seize compound
• 15 minutes on jack stands

Install

1. Let the exhaust cool. Really cool. 2. Support the van properly. Jack stands, wheel chocks, hazard lights on. 3. Unplug the downstream O2 sensor at its harness connector. 4. Remove the O2 sensor from the bung with the 22mm socket. Careful with the wire. 5. Thread the spacer into the bung, snug it up. 6. Apply a light coat of anti-seize on the sensor threads. 7. Thread the O2 sensor into the spacer, snug. 8. Reconnect the harness. 9. Clear the code, drive it.

Result

Trims settled, downstream O2 stopped mirroring upstream, and the P0420 hasn't returned in 2,000+ km. If the underlying cat eventually fails outright, this is a bandage — not a permanent fix. But for now, we're back on the road for the cost of a large pizza.

Full parts list on our Recommended Parts page.

What's next

Next up: the day the temp gauge crept higher than we liked. Chapter 8 — Thermostat & Cooling.