What causes fault code P0042?

• Most likely: Failed heater element inside the oxygen sensor (Bank 1, Sensor 3) • Likely: Damaged wiring or frayed harness shorting to ground/power • Possible: Corrosion or poor pin tension in the sensor connector • Rare: Faulty O2 heater relay or internal ECM failure

Powertrain (Engine & Gearbox)Standard EOBD

P0042 - HO2S Heater Control Circuit (Bank 1, Sensor 3)

Q: What are the symptoms of fault code P0042?

• Check Engine Light (MIL) illuminated • Slightly increased fuel consumption • Failed emissions test at inspection • Higher emissions during engine warm-up phase

Q: How do you fix fault code P0042?

STEP 1: Inspect the connector for Bank 1, Sensor 3 for signs of water ingress, corrosion, or oil contamination. Clean with contact cleaner if necessary. STEP 2: Use a multimeter to measure the resistance (Ohm) of the heater element across the sensor pins (typically the two wires of the same color). Normal resistance is usually 5–20 Ohms. 'O.L' or infinite resistance indicates a dead element. STEP 3: Verify 12V supply to the heater circuit with the ignition on. Also check the PCM-controlled ground side for continuity. STEP 4: If the heater element is open-circuit, replace the HO2S. Clear DTCs and perform a drive cycle. NOTE: This code will typically lead to an emissions test failure at vehicle inspection.

Last updated:1 February 2026

Related:Oxygen Sensor (Lambda), Wiring Harness, Engine Control Module (ECM), Oxygen Sensor

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0000

No Diagnostic Trouble Codes (DTC) Present

P0001

Fuel Volume Regulator Control Circuit / Open

P0002

Fuel Volume Regulator Control Circuit Range/Performance

P0003

Fuel Volume Regulator Control Circuit Low

Technical Description

DTC P0042 refers to a malfunction within the heater control circuit of the Heated Oxygen Sensor (HO2S) located at Bank 1, Sensor 3. The heater is vital for bringing the sensor element to its operational temperature rapidly. The ECM monitors the heater circuit's duty cycle and current draw. If the ECM detects an open circuit, high resistance, or a short circuit in the heater feed or ground return, the code is stored. This sensor is typically a post-catalyst sensor used for monitoring emissions efficiency.

Possible Causes

Failed heater element inside the oxygen sensor (Bank 1, Sensor 3)

Damaged wiring or frayed harness shorting to ground/power

Corrosion or poor pin tension in the sensor connector

Faulty O2 heater relay or internal ECM failure

Symptoms

Check Engine Light (MIL) illuminated

Slightly increased fuel consumption

Failed emissions test at inspection

Higher emissions during engine warm-up phase VIDA.

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P0042 - HO2S Heater Control Circuit (Bank 1, Sensor 3)

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P0042 - HO2S Heater Control Circuit (Bank 1, Sensor 3)

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