Engine Coolant Temperature Sensor Guide for 2002-2007 Mercedes-Benz S350
Understand the symptoms of a failing coolant temperature sensor and learn how to choose the right replacement for your S350.
- A bad ECT sensor can cause overheating, poor fuel economy, a check engine light, and rough running.
- The sensor is located at the front of the engine, but be warned: the old plastic part often breaks during removal, complicating the repair.
- There are no specific recalls or TSBs for this part on the S350, meaning failures are due to normal wear.
- For best results, choose a high-quality aftermarket brand like Delphi or Febi to ensure reliability and proper function.
Is Your S350's Engine Coolant Temperature Sensor Failing?
A faulty Engine Coolant Temperature (ECT) sensor can cause a surprising number of issues with your Mercedes-Benz S350. Because the engine's computer relies on this sensor for so many calculations, a bad signal can lead to poor performance, bad fuel mileage, and even prevent the engine from cooling itself properly. Ignoring the symptoms can lead to more serious engine problems down the road.
Common Symptoms of a Bad ECT Sensor
If the ECT sensor on your S350 is failing, you will likely notice one or more of the following issues:
- Check Engine Light: This is often the first sign. The engine's computer will detect a problem with the sensor's signal and store a fault code. Common codes include P0115, P0116, P0117, and P0119. 🎬 Watch: How to diagnose a sensor fault code before replacement.
- Poor Fuel Economy: A sensor that is stuck reading "cold" will cause the computer to permanently run a rich fuel mixture, wasting gas.
- Engine Overheating: A sensor that doesn't read "hot" correctly may fail to activate the radiator cooling fans when needed, causing the engine to overheat, especially in traffic.
- Black Smoke from Exhaust: The rich fuel mixture caused by a bad sensor can lead to unburnt fuel being burned in the exhaust, resulting in visible black smoke.
- Erratic Temperature Gauge: The temperature gauge on your dashboard may swing wildly, read too high, or not move at all.
- Rough Idle or Misfiring: Incorrect fuel and timing adjustments can make the engine run poorly, especially when idling.
A Note on Replacement Difficulty
The ECT sensor on the W220 chassis is located at the front of the engine and held in by a simple clip. While it looks like an easy job, the original plastic sensor can become very brittle with age. It is common for the sensor to break apart during removal, leaving a piece stuck in the engine block. Removing the broken piece can be difficult and may require careful drilling, turning a simple repair into a much bigger job. Be prepared for this possibility before you begin.
🎬 See this walkthrough for replacing the sensor on a W220.Choosing the Right Aftermarket Sensor
For a critical-yet-inexpensive part like the ECT sensor, brand quality matters. While many options are available, they generally fall into a few tiers.
| Brand Tier | Brands | General Assessment |
|---|---|---|
| OE-Quality Aftermarket | Delphi, Febi | These brands are often original equipment suppliers to automakers and are known for high-quality parts that fit and function correctly. Delphi highlights a one-piece design to improve reliability. Choosing from this tier is a safe bet for reliability and longevity. |
| Standard Aftermarket | Vemo, Standard Ignition, Walker Products, Four Seasons, Facet | These brands offer a balance of price and quality. While generally reliable, quality can sometimes be inconsistent compared to OE-level brands. Reviews for some Vemo parts, for example, show cases of premature failure. |
| Economy Aftermarket | URO Parts, True Tech | These brands are positioned as budget-friendly alternatives to genuine parts. While they can work, they are often associated with a higher risk of premature failure or fitment issues. URO Parts does offer a 'Premium' line that claims to improve on original designs. |
Pro Tip: Given the low cost of even the highest quality aftermarket sensors and the potential for a difficult removal if the part breaks, it is highly recommended to choose a brand from the OE-Quality tier like Delphi or Febi. The small extra cost is worth the peace of mind.
Frequently Asked Questions
Where is the coolant temperature sensor located on my S350?
On the W220 S-Class with the V6 engine (M112), the coolant temperature sensor is located at the front of the engine, near the top, underneath the secondary air injection pump. It is a black, 2-pin sensor held in place by a small clip.
Are there any recalls for this part?
No. Searches of the NHTSA database for the 2006 Mercedes-Benz S350 do not show any recalls specifically for the engine coolant temperature sensor.
Can I replace the ECT sensor myself?
Yes, this is a DIY-feasible job for someone with basic mechanical skills, provided the old sensor does not break during removal. You will need to remove the engine cover and secondary air pump for access. Be aware that some coolant will spill when the sensor is removed.
Should I replace the thermostat at the same time?
It is a common practice to replace the thermostat when replacing the coolant temperature sensor, as they work together to regulate engine temperature and can sometimes cause similar symptoms. Since you will already be working on the cooling system, replacing both can save you from having to do a similar job twice.
🎬 Watch: How to replace the thermostat and change your coolant.Helpful Videos
Technical Specifications
OEM Part Numbers: 0009056102 0009050700 0005425118 A0009056102 A0009050700 A0005425118
Connector: 2-pin, oval shape. Sensor Type: NTC (Negative Temperature Coefficient).
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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