Engine Coolant Temperature (ECT) Sensor Circuit Low Input
In simple terms, the ECU is seeing very low voltage from the coolant temperature sensor, which usually means the computer thinks the engine is extremely hot or the sensor circuit is shorted to ground.
What the Coolant Temperature Sensor Does
The ECT sensor tells the ECU the engine coolant temperature so it can control:
- Fuel injection
- Glow plug timing
- Cooling fan operation
- Cold start enrichment
If the ECU sees bad data, it may:
- Run poorly when cold
- Turn the cooling fan on constantly
- Have hard starting
- Throw a Check Engine Light
Common Causes on the 2001–2006 T1N Sprinter
1️⃣ Faulty Coolant Temperature Sensor (Most Common)
The sensor itself fails internally.
Typical signs:
- Code returns quickly after clearing
- Temperature reading stuck very high or very low
Location:
- On the thermostat housing at the front of the engine.
2️⃣ Wiring Issue
A short to ground in the sensor wiring will trigger P0117.
Look for:
- Broken insulation
- Wire rubbed through on engine
- Oil contamination in connector
3️⃣ Corroded Connector
T1Ns often get coolant or oil contamination in the plug.
Check for:
- Green corrosion
- Loose pins
- Coolant inside connector
4️⃣ Rare: ECU issue
Very uncommon unless there is wiring damage.
Quick Test (5-Minute Check)
- Turn ignition ON
- Unplug the coolant temp sensor
- Clear the code
- Start engine
If the code changes to:
P0118 – High Input
Then the sensor is bad and the wiring is good.
Replacement Info
Typical replacement sensor:
- Mercedes Part: 0061534228
- Mopar Equivalent: 05149141AA
Cost: $15–$35
Replacement time: 5–10 minutes
Steps:
- Let engine cool
- Remove electrical plug
- Pull metal retaining clip
- Pull sensor out
- Install new sensor with O-ring
You may lose a small amount of coolant.
Pro Tip for T1N Owners
If the sensor fails, the ECU may think the engine is overheating and trigger:
- High idle
- Cooling fan running full speed
- Reduced power
✅ Most likely fix: Replace the coolant temperature sensor.