P0117

Engine Coolant Temperature Sensor Circuit 1 Low Input

P0117 is a generic OBD-II powertrain diagnostic trouble code: Engine Coolant Temperature Sensor Circuit 1 Low Input. It is logged by the engine control unit when the coolant monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0117
Group
Powertrain
System
Coolant
Severity
Warning (MIL on)
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What P0117 means

P0117 is set when the Engine Control Module (ECM/PCM) detects an abnormally low voltage on the Engine Coolant Temperature (ECT) Sensor 1 signal circuit for more than approximately five seconds. The ECT sensor is a negative-temperature-coefficient (NTC) thermistor: its resistance decreases as coolant temperature rises, which in turn lowers the signal voltage seen by the ECM. A circuit-low condition means the voltage has dropped below the minimum plausible threshold — equivalent to a coolant temperature reading above roughly 300°F (149°C) — indicating either a genuine overheating event, a short to ground on the signal wire, or a failed sensor.

Because the ECM relies on the ECT signal to calculate injector pulse width, ignition timing, idle speed, cooling fan operation, and air-conditioning enablement, a corrupted low-voltage reading causes the PCM to assume extreme heat and react defensively: cooling fans are commanded on continuously, the AC compressor is switched off, and the temperature gauge may become inoperative. The engine may run rich or exhibit rough idle as fuelling strategies are forced into a fixed default map. Left unresolved, the underlying cause — especially an actual cooling system fault — can lead to serious engine damage.

P0117 is the low-side companion to P0118 (ECT Sensor 1 Circuit High). Both share the same sensor and circuit; P0117 points toward a short to ground or a failed sensor that has lost its resistance entirely, whereas P0118 points toward an open circuit or short to voltage. Related performance and rationality codes include P0116 and P0128.

Common causes

Most-frequently reported root causes when P0117 is logged.

  • 1
    Defective ECT sensor (internal short reducing resistance to near zero)
  • 2
    Short to ground on the ECT signal wire between sensor and ECM/PCM
  • 3
    Corroded, spread, or damaged terminals at the ECT sensor connector
  • 4
    Damaged or chafed wiring harness causing signal wire contact with chassis ground
  • 5
    Loose or poor ground connection at the ECT sensor or ECM
  • 6
    Actual engine overheating driving the sensor into its out-of-range low-voltage region
  • 7
    Coolant level too low, causing sensor to read air temperature instead of liquid
  • 8
    Faulty ECM/PCM (rare)

Symptoms drivers notice

Check Engine Light (MIL) illuminated with P0117 stored
Cooling fans running continuously regardless of actual engine temperature
Air conditioning compressor disabled or intermittently cut out
Poor fuel economy due to enriched default fuelling map
Rough idle or hesitation as ignition timing and fuelling revert to limp defaults
Coolant temperature gauge reading absent, pegged high, or erratic

How to diagnose P0117

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and confirm P0117 is stored; note any freeze-frame data and check for co-stored codes such as P0116, P0118, or overheating codes that may indicate an actual cooling system fault
  2. 2
    Compare the scan tool's reported coolant temperature against a contactless infrared pyrometer reading at the thermostat housing — a discrepancy greater than 10°F (5°C) points to a sensor or wiring fault rather than genuine overheating
  3. 3
    Visually inspect the ECT sensor connector and wiring harness for corrosion, spread terminals, chafing, or contact with hot or sharp components; repair any findings before continuing
  4. 4
    With the ignition off, disconnect the ECT sensor connector and measure the signal circuit voltage and resistance back to the ECM — a near-zero resistance or voltage at the signal pin confirms a short to ground in the harness
  5. 5
    With the connector unplugged, measure ECT sensor resistance across its terminals using a DMM and compare against the manufacturer's temperature-to-resistance table; a reading of near zero ohms at ambient temperature confirms an internally shorted sensor requiring replacement
  6. 6
    After repairing the fault, clear the DTC, perform a warm-up drive cycle, and confirm the coolant temperature reading tracks smoothly from cold start to operating temperature without re-setting P0117
  7. 7
    If the code returns with no wiring or sensor fault found, test the ECM reference voltage and ground circuits per the vehicle wiring diagram; ECM failure is rare but possible

Related powertrain codes

Frequently asked questions

Can I drive with P0117 active?

Short distances at low load may be possible, but it is not advisable. The ECM cannot accurately gauge coolant temperature, so it will disable the AC, run fans continuously, and apply a default fuelling map that increases fuel consumption. More importantly, if the code is triggered by genuine overheating rather than a wiring fault, continuing to drive risks serious engine damage. Have the vehicle inspected promptly.

Will P0117 cause a failed emissions test?

Yes, in most regions. Because the MIL is illuminated and a confirmed fault is stored, the vehicle will fail an OBD-II emissions inspection. The code must be repaired and the readiness monitors must complete a full drive cycle before retesting.

Is P0117 the same as the coolant temp sensor being bad?

Not always. A failed sensor is the most common hardware cause, but P0117 specifically indicates a low-voltage (short-to-ground) condition on the sensor circuit. A broken wire shorting to chassis ground will produce the same code even with a perfectly good sensor, so wiring and connector integrity must be verified before replacing the sensor.

How does P0117 differ from P0118?

P0117 (Circuit Low) means the ECT signal voltage is below the minimum expected range, which the ECM interprets as an impossibly high temperature — typically caused by a short to ground or a sensor whose resistance has collapsed. P0118 (Circuit High) means the voltage is above the maximum expected range, interpreted as an impossibly cold temperature — typically caused by an open circuit, a broken wire, or a sensor whose resistance has risen to infinity. Both codes disable the same protective responses but point to opposite electrical faults.

Disabling P0117 in software

RaceTune can permanently disable P0117 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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