ECT circuit reading below minimum voltage.
P0117 — also written PO117, P 0117, 0117 — is the OBD-II code for engine coolant temp sensor low input. Same code, same fix: this page covers every spelling of it.
Engine coolant temperature (ECT) sensor circuit reading abnormally low — the ECU thinks the engine is arctic-cold even when it's warm. On UK cars it's almost always a corroded ECT sensor connector, a split sensor body, or a broken wire between the sensor and the ECU — not the thermostat, and rarely the sensor itself failing electrically.
P0117 is a sensors fault code, typically costing £35–£200 to put right in a UK garage.
What P0117 actually feels like to drive
- Cold-start rich running, black exhaust smoke and eggy smell for the first 30 seconds
- Fuel economy drops 4–8 MPG because the ECU stays in cold-start enrichment map
- Heater blows cool air for a mile or two even when the temperature gauge shows normal
- Cooling fans run constantly (some ECUs treat sensor-low as a safety default = fans on)
- Temperature gauge either sits pinned cold or reads erratically
- Fault light comes on within 2–5 minutes of a cold start, clears after a warm-up on some cars
- On stop-start cars: engine refuses to auto-stop until the light is cleared
What causes P0117?
- Failed ECT sensor
- Short to ground
UK vehicles that throw P0117 most often
- Vauxhall Corsa / Astra / Zafira 1.4 / 1.6 (Z14XEP, A14NET, A16LET) — sensor housing cracks on the thermostat body
- Ford Focus / Fiesta / Mondeo 1.6/1.8/2.0 (Duratec, Zetec-SE) — corroded ECT connector by the coil pack
- VW Golf / Polo / Audi A3 1.2/1.4/1.6/1.9 TDI — the classic G62 sensor with the green (or black) plastic body that goes brittle at 80k+ miles
- BMW 1 / 3 Series N42/N46/N43 petrol — coolant sensor on the head, single-connector G-plug
- Mini Cooper 1.6 N12/N14/N18 — sensor in the thermostat housing, common leaker
- Peugeot 207 / 308 / Citroën C3 / C4 1.4/1.6 VTi (Prince EP6) — same Mini Cooper sensor design
- Nissan Micra / Note / Qashqai 1.2/1.6 — HR12/HR16 engine, sensor at rear of head
- Renault Clio / Megane / Scenic 1.2/1.6 16V — connector corrosion behind the inlet manifold
How P0117 is fixed
- Replace ECT sensor
- Repair wiring
The P0117 misdiagnosis that wastes money
P0117 is the code every parts-cannon garage loves — read the code, sell you a £45 sensor, hand back the car, and it's back inside a week with the light on again. Bob's 32 years on the tools rule: NEVER fit an ECT sensor until you've done two checks that cost nothing. One — unplug the sensor connector with the ignition on and read the voltage across the two pins with a multimeter: it should sit at exactly 5.0V. If you see 0V or 12V, the fault is in the wiring back to the ECU, not the sensor. Two — plug the sensor back in and back-probe the signal wire with the engine cold: at 20°C ambient you should see roughly 3.5–4.0V dropping smoothly to 0.5V as the engine warms. If the voltage jumps around, drops suddenly to zero, or sits pinned at 5.0V, THEN the sensor is genuinely faulty. On Ford Focus 1.6 and 1.8 Duratec the fault is almost always the connector pins corroding green from a coolant weep at the sensor O-ring — clean and dielectric-grease the pins and you'll fix half of them without touching the sensor. On VAG G62 sensors, always fit the updated part with the black locking clip (never the old green one) and replace the O-ring at the same time. On Mini Prince engines, the sensor and the thermostat housing come as one assembly — don't buy just the sensor because you'll be in there twice.
What a UK garage should charge for P0117
UK independent garage: diagnostic + live-data ECT trace £45–£70, sensor connector clean and dielectric grease £25–£45, ECT sensor replacement £45–£110 fitted depending on access (some are 5-minute jobs, some need the inlet manifold off), thermostat + sensor combined housing (Mini, VAG 1.4 TSI, Ford EcoBoost) £180–£340 fitted, wiring loom repair £60–£140 depending on the break location. Main dealer: 40–60% more. If a garage quotes £250+ for 'sensor and diagnostic' without showing you the voltage reading first, get a second opinion.
P0117 repair cost in the UK
Most P0117 repairs land between £35–£200 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0117
Before you spend anything, do the poor-man's diagnostic at home: plug in a £15 OBD reader with live-data (Torque app on Android, OBD Fusion on iPhone) and watch the coolant temperature parameter from cold start through to warm. A healthy sensor rises smoothly from ambient (say 12°C) to 88–92°C over about 8–10 minutes with no sudden drops or plateaus. If the reading jumps between -40°C (the default 'no signal' value on most ECUs) and something plausible, the fault is intermittent — usually a broken wire that opens up when hot, or a connector that lifts as the engine expands. Wiggle-test the connector and the loom near the sensor with the engine running and the light triggered — if the reading changes, you've found it. On Vauxhall Corsa D and Astra J with the A14NET turbo, we see the whole thermostat housing crack around the sensor boss — coolant weeps down the back of the block, sensor sits in a puddle, and P0117 stores intermittently. Replace the housing (not just the sensor) with a genuine or Behr part, use G13 pink coolant, and re-bleed the system properly through the header tank bleed screw. Not sure if your P0117 is a £15 wiring fix or a £340 housing job? <a href="/ask-a-mechanic-uk?code=P0117">Ask Bob about your P0117</a> for £14.99 with make, model, mileage and any paired codes — cheaper than one wasted sensor.
Can you drive with P0117?
Yes but cold-start will be rough.
Urgency: medium.