P2AFF

Code

Generic P — Powertrain

Engine Coolant Temperature Sensor 4 Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 26 EN: 59 RU: 27

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Page language: EN

Causes

Open or intermittent connection in the ECT sensor 4 harness
Short to battery/ignition voltage on the sensor circuit
Corroded, bent or pushed-out sensor connector terminals
Failed ECT sensor 4 (thermistor out of spec)
Poor or missing sensor ground or reference feed
Water/dirt intrusion in connector or harness

Symptoms

Check Engine Light (MIL) illuminated
Engine may run rich, have poor fuel economy or rough idle (PCM may use default cold/backup strategy)
Longer warm-up time or failure to enter closed-loop operation
Hard cold start or excessive cranking enrichment
Possible temperature gauge reading incorrect or pegged high/low depending on circuit design

What to check

Read freeze frame and live data with a scan tool; note ECT sensor 4 value and compare with other coolant temp sensors (if present)
Visually inspect sensor 4 connector, pins and harness for damage, corrosion or contamination
Backprobe sensor connector; with ignition ON (engine OFF) check reference voltage and ground at harness connector
Measure sensor 4 signal voltage with key ON and during warm-up; observe changes as engine warms
Unplug sensor 4 (if safe) to see if PCM response/voltage changes (use vehicle-specific procedure)
Check continuity and resistance of wiring between sensor connector and PCM

Signal parameters

Typical sensor circuit reference voltage: ~5 V (varies by manufacturer) — confirm vehicle-specific value
Typical signal voltage range: ~0.1–4.9 V (depends on circuit and temperature)
Cold engine: signal often near the high end (closer to reference) for thermistor-type sensors; hot engine: lower voltage
Sensor resistance behavior: NTC thermistor resistance decreases as temperature increases — verify with vehicle spec table

Diagnostic algorithm

Use scan tool: record freeze frame, pending codes, readiness and live data for ECT sensor 4 and other coolant sensors.
Visually inspect sensor 4 connector and wiring for damage, corrosion, heat damage, pin push-out or moisture. Repair as needed.
With connector connected, backprobe the connector with ignition ON (engine OFF): verify PCM reference voltage (usually ≈5 V) and good ground.
Measure the signal voltage at the sensor connector. A voltage near reference or stuck high indicates open/short-to-voltage; compare to expected values.
With engine cold then warm, observe live data: proper ECT sensor should change voltage/resistance as temperature changes. No change suggests open circuit or failed sensor.
If signal is high, disconnect the sensor and measure resistance across the sensor terminals (on bench or at connector) and compare to spec or expected NTC behavior (resistance should decrease as temperature increases). If sensor values are abnormal, replace the sensor.
Check continuity between sensor signal terminal and PCM pin; check for short to battery/ignition voltage on the signal wire and for open to ground. Repair any damaged wiring.
After repairs or sensor replacement, clear codes, run engine to operating temperature and confirm the code does not return and engine management returns to normal operation.
If all wiring and sensor check good and code persists, consider PCM fault and perform manufacturer procedures for module diagnosis or replacement.

Likely causes

Open or disconnected sensor connector/wiring
Sensor wiring shorted to battery voltage
Failed ECT sensor 4
Corroded or damaged connector terminals

Fault status

⚠️ Status

PCM detected high voltage on Engine Coolant Temperature Sensor 4 circuit (Circuit High). Possible open circuit, short to battery voltage, bad sensor or wiring/connector issue. DTC stored and fault may be pending or current.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours