P1118 Code: Diagnostic Guide
Complete technical reference for mechanics and DIY enthusiasts
What is the P1118 Code?
The P1118 code is a generic OBD-II trouble code that indicates an intermittent fault in the engine coolant temperature (ECT) sensor circuit, specifically for sensor 2 in bank 1. This code is commonly found in European vehicles, particularly those from Mercedes-Benz, BMW, Audi, and Volkswagen.
Technical Definition: P1118 is defined as “Engine Coolant Temperature Sensor 2 Circuit Intermittent Bank 1” according to SAE standards. The code is set when the Engine Control Module (ECM) detects irregular voltage fluctuations or signal dropouts from the secondary coolant temperature sensor.
How the Cooling System Monitoring Works
Modern vehicles typically have two coolant temperature sensors:
- ECT Sensor 1: Primary sensor that sends data to the ECM for fuel mixture and ignition timing calculations
- ECT Sensor 2: Secondary sensor used for dashboard temperature gauge and fan control module
The ECM continuously monitors both sensors and compares their readings. If Sensor 2 shows implausible values, intermittent signal, or values that don’t correlate with Sensor 1, the P1118 code is stored.
Symptoms of P1118 Code
When your vehicle’s computer detects the P1118 code, you may experience one or more of the following symptoms:
- Illuminated Check Engine Light: The most common and obvious symptom
- Engine Overheating: Due to improper cooling fan operation
- Erratic Temperature Gauge: Fluctuating or inaccurate readings on dashboard
- Cooling Fans Running Constantly: Or not activating when needed
- Reduced Engine Performance: The ECM may implement limp mode to protect the engine
- Poor Fuel Economy: Incorrect temperature readings lead to suboptimal fuel mixture
- Difficulty Starting: Especially when the engine is warm
Warning: Continuing to drive with an overheating engine can cause severe damage including warped cylinder heads, blown head gasket, or complete engine failure. Address the issue promptly.
Common Causes of P1118 Code
The P1118 code can be triggered by several issues. Here are the most common causes in order of probability:
- Faulty Coolant Temperature Sensor 2: The sensor itself may be malfunctioning and sending incorrect or intermittent signals to the ECM.
- Wiring Issues: Damaged, frayed, or corroded wires in the sensor circuit can cause intermittent signals. This is especially common near connectors or where wiring passes near hot components.
- Poor Electrical Connections: Loose, corroded, or oxidized connectors can disrupt the signal between the sensor and ECM.
- Failed Electronic Thermostat: Some modern vehicles use electronic thermostats that can fail and trigger this code.
- Low Coolant Level: In some cases, low coolant can cause temperature reading issues or air pockets that affect sensor operation.
- Faulty Engine Control Module (ECM): Although rare, a malfunctioning ECU could be the cause.
- Cooling System Air Pockets: Improperly bled cooling system can cause erratic temperature readings.
Technical Deep Dive: Sensor Operation
Most coolant temperature sensors are negative temperature coefficient (NTC) thermistors. Their resistance decreases as temperature increases. The ECM supplies a reference voltage (usually 5V) and measures the voltage drop across the sensor.
-40°C (-40°F) = 100,700 ohms ±4%
20°C (68°F) = 12,500 ohms ±1.5%
80°C (176°F) = 1,800 ohms ±2.5%
100°C (212°F) = 900 ohms ±3%
Diagnosis and Repair Procedures
Diagnostic Steps
Professional mechanics typically follow these steps to diagnose a P1118 code:
- Code Verification: Use an OBD-II scanner to confirm the code and check for additional codes
- Freeze Frame Data Analysis: Examine conditions when the code was set (engine temp, RPM, vehicle speed)
- Visual Inspection: Check wiring, connectors, and sensor for obvious damage or corrosion
- Live Data Monitoring: Observe sensor readings while engine warms up
- Resistance Testing: Measure sensor resistance at different temperatures
- Voltage Testing: Check for reference voltage and ground at sensor connector
- Wiggle Test: Gently manipulate wiring while monitoring live data to find intermittent faults
- Coolant System Inspection: Check coolant level, condition, and for air pockets
Repair Procedures
Depending on the diagnosis, repairs may include:
- Sensor Replacement: Drain coolant, disconnect electrical connector, remove old sensor, install new sensor with proper seal, refill coolant
- Wiring Repair: Locate damaged section, cut out damaged portion, solder and heat shrink new segment
- Connector Service: Clean corrosion with electrical contact cleaner, apply dielectric grease
- Coolant System Bleeding: Follow manufacturer procedure to remove air pockets
- Thermostat Replacement: Drain coolant, remove housing, replace thermostat and gasket
- ECM Replacement/Reprogramming: As a last resort after all other possibilities eliminated
Pro Tip: Always use OEM-quality sensors for replacement. Aftermarket sensors often have different calibration curves that can cause persistent issues.
Repair Cost Estimation
| Repair | Parts Cost | Labor Cost | Total Estimate |
|---|---|---|---|
| Coolant Temperature Sensor Replacement | $50 – $150 | $100 – $200 | $150 – $350 |
| Wiring Harness Repair | $20 – $50 | $100 – $300 | $120 – $350 |
| Electronic Thermostat Replacement | $150 – $400 | $200 – $400 | $350 – $800 |
| Coolant System Flush | $50 – $100 | $80 – $150 | $130 – $250 |
| ECM Reprogramming | $0 – $100 | $100 – $200 | $100 – $300 |
Note: Costs can vary significantly based on vehicle make and model, as well as local labor rates. Luxury vehicles typically cost more to repair.
Frequently Asked Questions
It is not recommended to drive with a P1118 code, especially if you’re experiencing overheating symptoms. Continuing to drive with an overheating engine can cause severe damage like warped cylinder heads or a blown head gasket, leading to much more expensive repairs. If your temperature gauge is reading normal, you might drive for a short time to reach a repair facility, but it’s best to address the issue immediately.
If you have intermediate automotive repair skills and proper tools, you may be able to address a P1118 code yourself. Simple fixes like replacing a sensor or repairing obvious wire damage might be DIY-friendly. However, diagnosis can be tricky due to the intermittent nature of this code, so professional help is often recommended. You’ll need a multimeter, basic hand tools, and possibly a scan tool capable of reading live data.
P1118 specifically indicates an intermittent circuit problem with the secondary coolant temperature sensor, rather than a complete failure. Other codes like P0115-P0118 indicate specific circuit malfunctions (open circuit, short to ground, etc.) for the primary sensor. The “intermittent” nature of P1118 makes it more challenging to diagnose as the problem comes and goes. P0118 would indicate a consistent fault, while P1118 suggests a sporadic one.
Yes, in most regions, any check engine light (including one triggered by a P1118 code) will cause your vehicle to automatically fail emissions testing. The code must be diagnosed and repaired, then the system must be reset and complete a drive cycle before testing. Some areas allow one ready monitor to be incomplete, but most require all systems to be ready for testing.
To prevent recurrence of P1118:
- Use high-quality OEM or equivalent replacement parts
- Ensure proper coolant mixture and level maintenance
- Address cooling system issues promptly
- Protect wiring from heat and abrasion with loom or tape
- Use dielectric grease on electrical connections
- Follow proper bleeding procedures after cooling system service
- Perform regular cooling system inspections as part of maintenance
