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P1289 Ford — Cylinder Head Temperature Sensor Circuit High Input

Moderate

Quick answer

P1289 means the cylinder head temperature (CHT) sensor on your Ford is sending a voltage too high to be real — an open circuit, which the computer reads as a wildly wrong temperature. Expect the A/C to be disabled as a precaution. The sensor and its heat-baked wiring are the usual culprits.

What it means

Many Fords — the modular 4.6L/5.4L V8s, the Focus, the 3.5L V6s among them — don’t read coolant temperature the usual way. Instead they use a cylinder head temperature (CHT) sensor threaded into the head’s metal itself, which lets the computer detect overheating even if the coolant is gone. The sensor is a thermistor on a 5-volt reference: its voltage falls as the engine warms. P1289 sets when that voltage sticks near the top of the range (above roughly 4.6 volts) — electrically, an open circuit, a reading no real engine temperature could produce.

With the signal lost, the computer falls back on protective assumptions. Depending on model and strategy it may read the impossible voltage as a deep-cold −40° or treat it as a potential overheat — and the visible result of the fail-safe is often a disabled air conditioner, a misbehaving temperature gauge, harder cold-engine manners and worse fuel economy from running a cold-engine fuel map.

The cause is almost always physical and close to the sensor: the sensor’s own internals fail with heat age, and its connector and wiring live in one of the hottest places on the engine, where insulation goes brittle and pins corrode. One honest warning before parts: on several Ford engines (the 5.4L V8 notably) the sensor hides under the intake manifold, so confirming the diagnosis before tearing in matters more than usual.

P1289 symptoms: what you'll notice

  • Check engine light, sometimes with no other obvious change.
  • Air conditioning that stops working or cuts in and out — the computer sheds the A/C load when it suspects overheating.
  • Temperature gauge reading wrong: stuck cold, pegged hot, or erratic.
  • Worse fuel economy and rich-smelling exhaust, from the computer running a cold-engine fuel map.
  • Cooling fans running constantly on some models, as another fail-safe.

Common causes

Ordered from most to least likely.

  1. 1.

    Failed CHT sensor

    The most common ending — the thermistor opens internally with age and heat cycles.

  2. 2.

    Broken, chafed or heat-brittled wiring to the sensor

    The harness bakes against the head — internal wire breaks are common and invisible from outside.

  3. 3.

    Corroded or backed-out connector pins

    Free to inspect, and a frequent fix — moisture plus heat cycling loosens this connection.

  4. 4.

    PCM fault

    Rare — only after the sensor and the full circuit measure good.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Read the sensor in live data

    Key on, look at the CHT temperature PID. A reading of −40° (or an impossible value) confirms the open circuit this code describes. A plausible temperature with P1289 stored historically suggests an intermittent — wiggle-test the harness while watching.

  2. 2 Inspect the connector first

    Find the CHT sensor connector (location varies — on some engines it’s accessible, on others the sensor is under the intake but the connector can still be reached). Look for green corrosion, melted plastic and backed-out pins. Clean, reseat, clear the code and retest — the free fix when you get one.

  3. 3 Test the circuit before condemning the sensor

    With the connector unplugged, key on: you should see the 5-volt reference on one harness pin. Briefly bridging the two harness pins should swing the live-data reading to the opposite extreme — if it does, the wiring and computer are good and the sensor is condemned; if not, chase the harness toward the PCM.

  4. 4 Measure the sensor itself

    Measure resistance across the sensor’s pins and compare against the spec table for your engine. An open (infinite) reading at any temperature confirms a dead sensor.

  5. 5 Replace with the access job priced in

    The sensor itself is inexpensive (use Motorcraft/OEM). On engines where it sits under the intake manifold, the labor dwarfs the part — which is exactly why steps 1–4 come first. While in there, inspect the section of harness near the sensor and repair any brittle insulation.

Parts & tools you may need

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Related codes

Frequently asked questions

What does code P1289 mean?
P1289 means the cylinder head temperature (CHT) sensor on your Ford is sending a voltage too high to be real — an open circuit, which the computer reads as a wildly wrong temperature. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.
Can I drive with P1289?
Briefly and gently, yes — but understand what you’ve lost: the computer can no longer see engine temperature, which is its early-warning system against overheating damage. With the sensor blind, an actual cooling problem could go unannounced. Treat it as a this-week repair, and watch the gauge habits you still trust.
Why did my A/C stop working with this code?
That’s the fail-safe, not a coincidence. When the computer can’t trust the head temperature reading, it assumes the worst and sheds the A/C compressor load to protect the engine. Fix the sensor circuit and the A/C returns — there’s usually nothing wrong with the A/C itself.
Is a CHT sensor the same as a coolant temperature sensor?
Same job, smarter placement. A conventional ECT sensor reads the coolant; Ford’s CHT sensor reads the metal of the head itself, so it still reports useful temperature even if the coolant has leaked out — which is exactly when you need the warning most. They are different parts and not interchangeable; order the CHT sensor for your engine.
My temperature gauge went crazy — same problem?
Very likely. On many of these Fords the dash gauge is driven from the CHT signal, so an open circuit pegs it cold, hot, or erratic depending on model. A gauge acting possessed plus P1289 is the circuit talking, not the gauge — fix the sensor side first.
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