P1165

Code

SCION P — Powertrain

Air-fuel ratio sensor heater circuit malfunction

Brand: SCION

Type: P — Powertrain

Views: UK: 16 EN: 20 RU: 14

AI status

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Page language: EN

Causes

Blown fuse or faulty heater relay (if used)
Open, shorted or high-resistance wiring between ECM and A/F sensor heater
Corroded, loose or water-damaged sensor connector
Failed A/F sensor heater element
Faulty ECM heater driver or internal short
Aftermarket sensor or incorrect replacement part (wrong type/resistance)

Symptoms

Malfunction Indicator Lamp (MIL) illuminated with stored DTC P1165
Longer warm-up time for closed-loop operation (poor fuel economy until warm)
Elevated emissions or failed emissions test
Engine may run rich/lean briefly during warm-up or have unstable idle until sensor reaches operating temp

What to check

Read and record freeze-frame and all stored codes with scan tool
Visually inspect A/F sensor connector and wiring for corrosion, damage, or water intrusion
Check related fuses and heater relay (if equipped)
Measure heater element resistance at the sensor connector with harness disconnected
Backprobe connector with scan tool/oscilloscope while commanding heater ON to verify supply voltage and ground drive
Wiggle harness and connector while monitoring for intermittent changes

Signal parameters

Heater supply voltage: near battery voltage (approx. 11–14 V) when ECM commands heater ON
Heater element resistance: typically low—commonly a few ohms to tens of ohms (consult vehicle-specific spec); open circuit indicates failed heater
ECM command: PWM or switched 12 V (or ground-side switching depending on vehicle); verify with scope or backprobe
No-voltage condition when heater not commanded (ignition ON but heater OFF)

Diagnostic algorithm

Connect a scan tool and confirm P1165 and any related codes; note freeze frame data and readiness status
Perform visual inspection of the sensor, connector, and harness for corrosion, pin damage, melted insulation, or water entry
Check fuses and relays that supply the heater circuit; replace if blown and retest
With ignition OFF, disconnect sensor connector and measure heater element resistance across heater pins; compare to vehicle specification (open or very high resistance = sensor fault)
With connector disconnected and ignition ON (engine OFF), check for battery voltage at the heater supply pin if the circuit is supply-switched; if voltage is present, suspect sensor or internal open
Command the heater ON with a scan tool (if supported) and backprobe to verify the ECM is providing the expected control signal (voltage or switched ground) and that current flows through the heater
If supply is present and resistance is correct but ECM does not command or drive the circuit, suspect ECM driver fault—confirm with wiring continuity checks to rule out harness issues before replacing ECM
Inspect and repair any damaged wiring, corroded connectors, or poor grounds. Replace the A/F sensor if heater element is out of spec or physically damaged
After repair, clear codes, run the vehicle through readiness conditions and verify heater operation and that the code does not return

Likely causes

A/F sensor heater element failed (open or very high resistance)
Blown fuse supplying the heater circuit or bad relay
Broken/shorted wire or poor pin contact at sensor connector
ECM heater control transistor failed or intermittent

Fault status

⚠️ Status

MIL ON — Diagnostic trouble code P1165 stored. A/F sensor heater circuit malfunction detected; emissions and closed-loop fuel control may be affected.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5 - 2.0 hours

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P1165 — Air-fuel ratio sensor heater circuit malfunction