P0420 Catalyst Efficiency Below Threshold on Smart Fortwo 451 / 453
P0420 says the catalytic converter isn't scrubbing exhaust gases as well as expected. On a Smart it's more often a lazy downstream O2 sensor or an exhaust leak ahead of the cat than a dead converter. Confirm what the sensors are doing with live data before quoting a catalytic converter.
Typical Symptoms
- Check engine light with P0420
- Little or no noticeable drivability problem
- Slight reduction in fuel economy
- Emissions test fail in OBD‑inspection states like CA or NY
What it means
The ECU monitors two oxygen sensors – one ahead of the catalytic converter and one after it – to judge how well the converter is working. A healthy converter smooths out the downstream sensor's voltage swings; a tired converter lets the swings mirror the upstream sensor. P0420 means the downstream reading looks too much like the upstream. On a Smart the usual fix is a sensor or a leak, not a converter.
Likely causes, cheapest first
- Downstream O2 sensor reading slow or failed. Sensors age and report false P0420 even when the cat is fine.
- Exhaust leak before the converter. A manifold gasket, flex pipe or downpipe joint leak lets outside air in and skews the readings.
- Upstream O2 sensor lazy. If the upstream sensor is slow, fuel trims drift and the cat can't do its job properly.
- Catalytic converter truly worn out. Last on the list; confirm with live sensor data before replacing.
DIY check steps
- Read any stored O2 sensor codes (P0130–P0155). Fix sensor codes before chasing the converter.
- Inspect the exhaust manifold and downpipe joints for sooty streaks, fresh black deposits or a ticking noise on cold start – signs of a leak.
- Use a scan tool or Bluetooth dongle to watch live O2 sensor data. A healthy upstream swings 0.1–0.9 V several times per second; a healthy downstream stays steady near 0.6–0.7 V. A downstream that mirrors the upstream is the classic dead‑cat pattern.
- Replace the downstream sensor if its reading is slow or stuck. If that doesn't fix it, replace the upstream sensor.
When to call a shop
If both sensors are fresh, there are no exhaust leaks and live data still shows the cat isn't working, it's time to consider a catalytic converter. Replacing the cat on a Smart can be done at home, but the exhaust manifold studs often seize and break, so many owners let a shop handle it. Outside emissions states some owners live with P0420 – just be aware of the emissions implications.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Downstream O2 sensor (post-cat) | $50-130 | Search Google |
| Upstream O2 sensor (pre-cat) | $60-160 | Search Google |
| Manifold-to-downpipe gasket | $15-35 | Search Google |
| Catalytic converter (aftermarket) | $200-500 | Search Google |
| Catalytic converter (OEM) | $700-1200 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Service manual on Manualslib — external mirror (we don't host this specific document).
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- Evilution: OBD-II error code reference
- FQ101: OBD code lookup
- OBD-codes.com: P0420 generic reference
Stuck on this one?
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