Will P0115 stop my car from starting?

Usually not — but it can make cold starts difficult. The engine still cranks, fuel still gets injected, and most platforms have a fail-safe default that assumes a middle-of-the-road coolant temperature when the sensor signal is unreliable. The problem is that fail-safe value is wrong for actual cold conditions, so the cold-start enrichment isn't right and the engine takes longer to catch. In freezing weather, with a worn battery, with marginal fuel, or on a high-mileage engine, that delay can stretch long enough to feel like the engine isn't going to start. Once the engine runs, it usually drives normally — the impact concentrates around cold-start moments and the warm-up phase.

What's the difference between P0115, P0117, and P0118?

Direction and specificity of the failure. P0117 sets when the ECT signal voltage is too low — usually indicating either an extremely cold reading that shouldn't be possible (like -40°F when ambient is 70°F) or a shorted-to-ground signal wire. P0118 sets when the signal voltage is too high — usually indicating an open circuit or sensor failure that produces a reading above the maximum plausible temperature. P0115 is the general 'circuit malfunction' code — used when the signal is unusable but doesn't cleanly fit the high or low pattern, often because it's intermittent or noisy. Diagnostically, all three end in either sensor replacement or wiring repair, but P0117 and P0118 give you a direction up front while P0115 forces you to read the live data to figure out which way the signal is failing.

Can a bad coolant temperature sensor affect fuel economy?

Yes, noticeably. When the ECT sensor signal is unreliable, the PCM typically defaults to a fail-safe coolant temperature value that's intentionally conservative — usually a moderate warm value. The result is that cold-start enrichment runs longer than it should and closed-loop fueling either fails to engage or engages incorrectly. Both conditions waste fuel. Drivers typically see a 1-3 MPG drop when an ECT fault is active, sometimes more in cold weather where the cold-start enrichment effect is amplified. The fix is usually cheap, and the fuel economy improvement alone often pays back the repair cost within a few tanks.

How do I test the coolant temperature sensor?

Two methods. With the engine cold (sat overnight), use a scan tool to read the ECT live data parameter. The reading should be close to ambient temperature — within a few degrees. A reading pinned at minimum (-40°F) or maximum (250°F+) with a cold engine is the signature of a failed sensor or wiring fault. Second, disconnect the sensor and measure resistance across the terminals with a multimeter. Compare to the resistance-vs-temperature spec for your platform. Most ECT sensors read several thousand ohms at room temperature, dropping to a few hundred ohms at operating temp. An open reading (infinite resistance) or a sensor far outside the expected range confirms it's bad. Both tests together — plus a connector inspection — usually pinpoint the fault without needing a shop.

OBD-II trouble code

P0115: Engine Coolant Temperature Sensor Circuit Malfunction

The PCM is reporting a generic circuit fault on the engine coolant temperature (ECT) sensor — the signal is unusable for fueling calculations. Because the PCM uses coolant temp to set cold-start enrichment, a bad signal can cause hard cold starts, rich running, or stalling shortly after startup.

Quick facts

System: Powertrain
Category: Temperature
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $50 – $400
DIY difficulty: Beginner DIY

What does P0115 mean?

The engine coolant temperature (ECT) sensor is one of the most important sensors on the engine for fueling and timing decisions. It's a thermistor — its resistance drops as temperature rises — and the PCM watches it to decide how much extra fuel to inject during cold start, when to lock out closed-loop fueling, when to enable EGR, when to turn on the cooling fan, and when to enable certain diagnostics. P0115 sets when the PCM detects a general circuit fault that can't be cleanly categorized as 'too low' (P0117) or 'too high' (P0118) — usually an intermittent or noisy signal that fails the basic sanity check.

In practical terms, P0115 covers the broad bucket of ECT failures: corroded connectors that drop in and out, broken sensors that produce nonsense readings, wiring chafe that creates erratic voltages, and rarely the PCM's input circuit itself. On some platforms the same code sets if the ECT signal disagrees with another temperature reference (like the cylinder head temperature sensor on certain Ford engines) by more than the allowable drift.

Drivers usually notice the symptoms before the code: hard cold starts (the engine cranks too long because the cold-start enrichment isn't happening), a faint fuel smell during warm-up, rough idle until the engine reaches operating temperature, occasional stalling when first starting from cold, and sometimes a cooling fan that runs constantly even when the engine is cold (because the PCM defaults to a fail-safe assumption of high coolant temp). On platforms with electric water pumps tied to the ECT reading, you may also see thermostat-related codes alongside P0115 because the cooling system control logic has lost its temperature reference.

The physical fault is almost always at the sensor or its connector. The ECT lives in a wet, hot, vibrating location, and the connector accumulates corrosion or oil seepage over years. Replacement is usually a 15-minute job on accessible platforms (most American V8s, most inline-fours), longer on engines where the sensor sits under the intake manifold or in a buried housing. The part itself is cheap — $15-50 — so the cost is mostly labor.

Common causes

Failed ECT sensor — thermistor element open or shorted internally (the dominant cause)
Corroded or contaminated ECT sensor connector
Coolant intrusion into the sensor connector (common on older vehicles with deteriorating seals)
Wiring chafe damage between the ECT sensor and the PCM
Open or shorted signal-return wire
Disagreement between ECT and a redundant temperature reading (CHT sensor on some Ford engines)
PCM signal-input fault (rare)
Aftermarket sensor of wrong specification — different curve than OEM

Symptoms

Check Engine Light on
Hard cold starts — engine cranks too long before catching
Rough idle until engine reaches operating temperature
Faint fuel smell during cold-start warm-up (rich running)
Cooling fan running constantly, even on a cold engine
Reduced fuel economy
Possible stalling shortly after a cold start
Temperature gauge in the dash may behave erratically

Diagnostic steps

1.Pull all codes. P0115 alone is the generic ECT fault case. P0115 with thermostat codes (P0128, P0125) means the cooling system control logic is also seeing trouble.
2.Use a scan tool to read the ECT live data value. With a cold engine, ECT should read close to ambient temperature (within a few degrees). A reading of -40°F (sensor maximum reading low) or 250°F+ (sensor maximum reading high) with the engine cold is the giveaway that the signal is broken.
3.Disconnect the ECT sensor connector and measure resistance across the sensor terminals with a multimeter. Compare to the resistance-vs-temperature spec for your platform. Out-of-spec or open confirms a failed sensor.
4.Inspect the connector for green corrosion, oil contamination, or pushed-back pins. Look for coolant seepage past the connector seal — a common failure mode on aged sensors.
5.Wiggle-test the harness from the sensor back along the routing path to identify any chafe damage producing intermittent failures.
6.On platforms with a redundant CHT (cylinder head temperature) sensor, compare ECT and CHT readings. Large disagreement points at one or the other being wrong.
7.Replace the sensor if it tests out of spec. Apply a thin coat of dielectric grease to the connector during reinstall to prevent future corrosion.
8.After replacement, clear the code, let the engine reach full operating temperature, and verify the ECT live data tracks engine warmup correctly.

Repair cost

$50 – $400

Sensor itself is $15-60. On accessible platforms (most American V8s, most inline-fours, many Japanese engines), total replacement is $80-200 with about 30 minutes of labor. On engines where the sensor sits under the intake manifold or in a buried housing (some V6 platforms, certain European designs), expect $200-400 because access labor dominates. Connector pigtail repair adds $50-150 if the connector itself is the problem rather than the sensor. This is a strong DIY candidate on most platforms because the part is cheap and the work is straightforward.

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DIY vs shop

This is a beginner-friendly repair. Common hand tools, a free afternoon, and a willingness to follow a procedure are usually enough. The risk of causing a bigger problem is low if you read up on your specific vehicle first.

Related codes

P0116 P0117 P0118 P0119P0125P0128P0001