P0135 Upstream O2 Sensor Heater on Smart Fortwo 451 / 453
P0135 is a fault in the heater element built into the upstream oxygen sensor. The heater warms the sensor up quickly so the closed-loop fuel system can come online. On a Smart Fortwo, this is almost always solved by replacing the sensor — wiring or fuse faults are rare.
Typical Symptoms
- Check engine light with code P0135
- Slightly worse fuel economy until the engine fully warms up
- Often paired with P0130 (sensor circuit malfunction) on the same sensor
What it means
Modern oxygen sensors do not work cold. The ECU runs the engine open-loop (fixed fuel maps) until the sensors are hot enough to read accurately, then switches to closed-loop where the sensors actually control the air-fuel ratio. To shorten that warm-up window, every modern O2 sensor has an internal heater element. P0135 means the ECU is not seeing the heater pull the current it should.
On a Smart Fortwo, this is almost always the heater element itself dying. The sensor is sealed and the heater wires are not separately serviceable, so the fix is sensor replacement. Before paying for a sensor, it is worth checking the connector and the heater-circuit fuse — both are free, and a corroded connector can cause exactly the same fault.
Likely causes, cheapest first
- Sensor connector loose or corroded. Free to inspect.
- Heater fuse blown. Cheap to test and replace.
- Sensor heater element failed internally. The dominant cause once the connector and fuse are ruled out. Replace the sensor.
- Wiring chafe along the harness. Rare. Inspect the harness routing where it passes any metal edges.
DIY check steps
- Check the sensor connector. Disconnect, look for corrosion, clean with electrical contact cleaner, push back firmly until it clicks.
- Check the relevant fuse for the heater circuit. The fuse panel diagram in your owner's manual or workshop manual identifies which one. A blown fuse is rare but free to confirm.
- Replace the sensor if the connector and fuse are clean. Use penetrating lubricant on the threads for a few days before removal — the sensor sits in hot exhaust and the threads seize. A 22mm ring spanner gives more grip and is less likely to slip than an open wrench.
- Apply anti-seize to the new sensor threads sparingly, torque to spec, reconnect, and clear the code.
When to call a shop
If a new sensor and a clean connector still leave P0135 stored, that is wiring or ECU territory. A shop with proper diagnostic equipment can scope the heater drive circuit and confirm whether the fault is in the harness or the ECU. This is rare — most P0135 cases on a Smart clear with sensor replacement alone.
If P0135 is stored alongside P0130, replace the sensor once and both should clear. They are different failure modes of the same part.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Upstream O2 sensor (Bank 1, Sensor 1) | $60-180 | Search Google |
| Sensor heater fuse (varies by model year) | $2-5 | 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: Roadster Lambda Sensors (DIY removal procedure)
- Evilution: OBD-II error code reference
- FQ101: OBD code lookup
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