P0113 and P0118: temperature sensors — cheap parts that cost you fuel

Two sensors that cost very little and fail quietly — but whose failure is visible every month on your fuel bill.

The coolant temperature sensor

Codes: P0117 (signal too low), P0118 (too high), P0125 (insufficient warm-up).

This is one of the most important sensors on the engine. Its reading determines:

• How much to enrich the mixture during warm-up.
• When to switch the cooling fan on.
• When to permit emissions system tests.
• Which ignition timing to use.

What happens when it lies. Reading 50 °F instead of 195 °F, the module believes the engine is permanently cold and enriches continuously. Result: consumption up 20–30 %, black deposits on the plugs, P0172 rich mixture, and a cooling fan that never runs. Rich mixture is covered in its own article.

The reverse case: reading 250 °F when cold — hard starting, because the mixture is too lean for a cold engine, and the fan runs constantly.

The most treacherous version: the sensor can lie within tolerance, and then there is no code at all. Consumption is up, no lamp, and the workshop finds nothing.

The fast check

Warm the engine and read the temperature in live data:

Warm reading	Conclusion
185–205 °F	Normal
160–175 °F, stable	Thermostat, see P0128
Below 120 °F	Sensor or wiring
Jumping around	Connector contact
Exactly −40 °F or 285 °F	Open or short circuit

That last row matters: −40 and the top of scale are not temperatures, they are the ends of the range. A steady value pinned at a limit means look at the wire, not the engine.

The intake air temperature sensor

Codes: P0112 and P0113.

Less critical but still influential. The module uses air temperature to calculate density, and therefore the actual mass of air entering the cylinder.

Failure symptoms are milder: slightly worse response, a small rise in consumption, occasional summer detonation. Often unnoticeable to the driver.

A quirk: on many cars this sensor is built into the mass airflow meter and not available separately. So P0113 frequently means replacing the airflow meter — that sensor is covered in its own article.

The fast check

On a cold engine that has stood overnight, intake air temperature should roughly match coolant temperature and the weather outside. A 35-degree discrepancy means one of the two is lying.

That is the simplest possible test of both sensors at once, and it takes five seconds.

Causes of failure

Identical for both:

1. The connector. Corrosion, heat damage, a broken latch. The most common.
2. A broken wire right at the sensor, from vibration.
3. The thermistor itself. Degrades over time and begins lying gradually.
4. Scale on the body of the coolant sensor — slows its response.
5. Oil contamination of the air sensor from crankcase ventilation.

Why to check these first

These are the cheapest parts of all those affecting fuel consumption. Before hunting for causes among injectors, oxygen sensors, or the catalyst, read the temperatures in live data. It is free and takes a minute.

That same step comes first in the guide to increased fuel consumption — and not by accident.

In short

• The coolant sensor governs enrichment, the fan, and system testing.
• When it lies, consumption rises 20–30 % with no warning lamp at all.
• Readings at −40 or the top of scale mean an open or short, not a temperature.
• On a cold car both sensors should read roughly the outside weather.
• The cheapest check among everything that affects consumption.

Download the app — coolant and intake air temperatures are read with an ELM327 adapter over Bluetooth SPP on Android.