P0118 Coolant Temp Sensor High Input – (What It Means and How to Fix It)

Cold start.
Dash light on.

Scan it.

P0118.

This one sounds technical.

It’s not.

It usually means the engine computer thinks the coolant temperature is way colder than reality.

And that throws everything off.

What this code means

P0118 means the ECU is seeing a high voltage signal from the coolant temperature sensor.

High voltage = low temperature reading.

So the ECU thinks:

• Engine is stone cold
• Even when it isn’t

The ECU uses coolant temp to control fueling, timing, and idle.

What the coolant temp sensor does

This sensor is simple, but critical.

It tells the ECU how hot the engine is.

The ECU uses that to:

• Adjust fuel mixture
• Control idle speed
• Turn cooling fans on and off
• Manage emissions

If the signal is wrong, the whole strategy is wrong.

Common symptoms

This one can show up in a few ways.

• Hard starting, especially when warm
• Rough idle
• Excess fuel consumption
• Black smoke from exhaust
• Cooling fans running when they shouldn’t
• Temperature gauge reading cold or erratic
• Check engine light

Engine behaves like it’s stuck in cold start mode.

Most common causes

Faulty coolant temperature sensor

Most common.

Sensor fails internally.

Sends incorrect high voltage signal.

Cheap part. Easy fix.

Wiring fault or open circuit

Very common with this code.

If the circuit is open:

• ECU sees maximum voltage
• Interprets it as extremely cold

Causes:

• Broken wire
• Corroded connector
• Loose plug

Poor connection at the sensor

• Dirty terminals
• Moisture in connector
• Loose fit

Intermittent faults are common here.

ECU or reference voltage issue

Less common.

But possible.

• Fault in ECU
• Issue with 5V reference supply

Only check this after ruling out the basics.

How to diagnose it

Check the sensor reading on a scan tool

Start here.

• Compare coolant temp to actual engine temp

If the engine is warm but reading is very low:

You’ve found your issue.

Inspect the connector

• Unplug it
• Check for corrosion
• Check for loose pins

Check wiring continuity

• Look for broken wires
• Check for open circuit

An open circuit will trigger this code.

Test the sensor

• Remove and test resistance
• Compare to spec at known temperatures

If it’s out, replace it.

How to fix it

Most fixes are straightforward.

• Replace coolant temperature sensor
• Repair wiring faults
• Clean or repair connector
• Rarely replace ECU

This is usually a quick job and not expensive.

Can you drive with P0118

Yes, but it’s not ideal.

• Engine may run rich
• Fuel economy drops
• Long term can damage catalytic converter

If it’s running badly, get it sorted.

FAQs

What does high input mean?

It means the ECU is seeing a higher than expected voltage, usually caused by an open circuit or failed sensor.

Can a bad sensor cause overheating?

Not directly, but it can affect fan operation and cooling strategy.

Why is my fan running all the time?

The ECU may be defaulting to a safe mode if it cannot trust the sensor reading.

Is this an expensive fix?

No. In most cases, it’s a low cost sensor or wiring repair.