If your Mercedes reports P011800 (Coolant Temperature Sensor 1 short circuit to positive), the engine management light will appear, and the vehicle may go into limp mode or show performance issues. This guide walks you through diagnosis, replacement, and clearing the fault using a professional scanner.
Table of Contents
Watch the Full Repair Video
Step-by-step video guide for fixing P011800 on Mercedes 2.1L diesel engines.
What is P011800?
Fault code P011800 indicates the engine control module (ECM) detects a short circuit (to positive) on the coolant temperature sensor (CTS) circuit. The ECM relies on accurate coolant temperature to control fuel and emissions — incorrect readings can trigger warnings or limit engine operation.
Common Causes
- Damaged wiring or insulation causing a short to positive voltage
- Corroded or bent connector pins
- Failed or aged coolant temperature sensor (CTS)
- Poor-quality aftermarket replacement parts
DIY Fix — Step-by-Step
- Safety first: Work on a cool engine, wear gloves and eye protection.
- Locate the sensor: On the 2.1 L Mercedes diesel, the CTS is near the thermostat housing — a small sensor with a single plug.
- Inspect wiring & connector: Look for damaged insulation, corrosion, or pushed/bent pins. Repair or replace wiring/connector as needed.
- Remove the sensor: Unplug the connector, release the retaining clip/pin, and pull the sensor out. Expect a small coolant leak — have a catch pan or lightly drain coolant if necessary.
- Install a quality sensor: OEM/genuine replacement is recommended for reliability.
- Scan & clear codes: After replacement, perform a full system scan and clear fault codes.
- Test drive & re-check: Confirm the check engine light stays off and that temperature readings are stable.
Recommended Tool
Use a professional scanner to read/erase the DTC and verify sensors. The iCarsoft CR ULTRA P is a powerful option — reads/clears codes, supports full-system diagnostics, and offers advanced service functions ideal for DIYers and small workshops.
View iCarsoft CR ULTRA PQuick Troubleshooting Tips
- If connector pins look green or white, clean them with electrical contact cleaner and inspect again.
- If the wiring harness is chafed near engine mounts, repair and protect with heat-shrink and conduit.
- Always fit a trusted-quality (preferably OEM) sensor — it reduces repeat failures.
Related Articles & Further Reading
Interested in other fault codes on the same engine? See our detailed guide on Mercedes P047214 – Exhaust Back Pressure Sensor 1 Short Circuit.
For further authoritative information on diagnostic trouble codes, refer to resources such as OBD-CodeList P0118.
FAQ
Q: Can I keep driving with P011800?
A: Not recommended. You may experience limp mode or incorrect engine behavior. Repair soon to prevent overheating or drivability issues.
Q: Will replacing the sensor clear the code automatically?
A: No. After replacement, you should clear stored DTCs with a diagnostic scanner such as the iCarsoft CR ULTRA P.
Q: Do I need to drain coolant to swap the sensor?
A: A small amount may leak when the sensor is removed. It’s best to work on a cool engine and be prepared to catch or top up coolant afterward.
Q: Is an aftermarket sensor OK?
A: Cheap sensors can fail sooner. OEM or high-quality replacements are recommended to avoid repeat visits.
