P0117 Coolant Temperature Sensor Low on Smart Fortwo 451 / 453
P0117 means the engine coolant temperature (ECT) sensor is reading lower than physically possible — almost always a connector or sensor problem rather than the engine actually being that cold. Inspect the connector first; sensor replacement is the usual fix.
Typical Symptoms
- Check engine light with code P0117
- Cold-start mixture richer than usual (mild rough idle, slight fuel smell)
- Cooling fan may run earlier or longer than usual
- Live data shows the ECT reading well below ambient temperature
What it means
The engine coolant temperature (ECT) sensor is a small thermistor that screws into the engine near the thermostat housing. As the coolant warms up, the sensor's resistance drops in a known curve, and the ECU uses that signal for cold-start enrichment, cooling fan control, idle control, and a long list of other decisions. P0117 sets when the ECU sees a voltage that maps to a temperature too cold to be real — for example, a Smart Fortwo sitting in a heated garage that suddenly reports -40°F.
On a Smart Fortwo, the dominant cause is a connector that has corroded, gone loose, or had a pin push back into the housing. The sensor itself is also a wear part, but replacement is rare without first checking the connector. Harness chafe near the thermostat housing has been seen occasionally and is worth a look if the connector is clean.
Likely causes, cheapest first
- Connector loose or corroded. Free to inspect. Push back firmly until it clicks. Clean any corrosion with electrical contact cleaner.
- Wiring chafed near the thermostat housing. Visual inspection. Look for cracked insulation or a wire rubbed against a metal edge.
- Sensor failed internally. Cheap part. Once the connector and harness are ruled out, replace the sensor.
DIY check steps
- Read the live ECT value with a Bluetooth OBD-II dongle. A reading well below ambient temperature confirms a real sensor or connector problem rather than a one-off code.
- Disconnect the sensor connector and inspect both sides. Green corrosion, oil ingress, or a pin pushed back into the housing all explain the fault.
- Reconnect firmly and clear the code. If it stays clear after a few warm-up cycles, the connector was the issue.
- Replace the sensor if the connector is clean and the code returns. The ECT screws into the engine near the thermostat — a 19mm or 22mm socket usually fits, depending on year. Drain a small amount of coolant first and have the new sensor ready to swap quickly.
- Top up coolant after replacement and bleed any air out as needed.
When to call a shop
Almost never necessary for P0117 alone. If a new sensor and clean connector still leave the code stored, the next step is harness diagnosis with proper equipment — a multimeter check at the ECU side of the wiring will isolate whether it's a wiring break or an ECU input fault. ECU-side faults on this circuit are very rare on a Smart Fortwo.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Coolant temperature sensor (ECT) | $20-60 | Search Google |
| Sensor connector / pigtail repair kit | $10-25 | 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: P0117 generic reference
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