P0042
HO2S Heater Control Circuit (Bank 1 Sensor 3)P0042 is a generic OBD-II powertrain diagnostic trouble code: HO2S Heater Control Circuit (Bank 1 Sensor 3). It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0042 means
P0042 is set when the powertrain control module (PCM) detects an abnormal condition in the heater element control circuit for the heated oxygen sensor (HO2S) located on Bank 1, Sensor 3 — the third oxygen sensor downstream from the engine on the side containing cylinder number one. The internal heating element within the HO2S allows the sensor to reach its minimum operating temperature of approximately 300–350 °C quickly after a cold start, ensuring accurate rich/lean voltage swings are available to the PCM for closed-loop fuel control and catalyst efficiency monitoring. The PCM supplies a switched ground or voltage to the heater circuit and monitors the resulting current draw; when the measured value falls outside the calibrated window — regardless of whether it is too high, too low, or simply absent — P0042 is stored and the MIL is illuminated. Unlike the upstream sensors used for active fuel trim, Sensor 3 is primarily a post-catalyst monitor, so drivability impact is usually subtle, but emissions compliance and the ability to detect catalytic converter failure are compromised until the fault is resolved.
Common causes
Most-frequently reported root causes when P0042 is logged.
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1
Failed HO2S heater element (open or shorted internal resistor)
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2
Blown fuse protecting the oxygen sensor heater supply circuit
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3
Open, shorted, or corroded wiring between the PCM and sensor heater terminals
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4
Water or corrosion intrusion at the sensor harness connector
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5
Wiring chafed against exhaust components causing intermittent short to ground or power
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6
Faulty PCM heater-control driver (rare, usually after all circuit checks pass)
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7
Aftermarket or incorrect replacement sensor with incompatible heater resistance
Symptoms drivers notice
How to diagnose P0042
A typical diagnostic flow when this code is present.
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1
Connect a scan tool, confirm P0042 is stored, and note any companion codes (P0043, P0044, catalyst efficiency codes)
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2
Inspect the HO2S Sensor 3 connector and wiring harness for corrosion, backed-out pins, chafing, or heat damage near the exhaust system
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3
Locate and test the heater circuit fuse; replace if blown and recheck to confirm root cause
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4
With ignition on, verify battery voltage is present at the heater power supply wire at the sensor connector
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5
Back-probe or use a breakout box to measure heater control wire for proper switched ground from the PCM
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6
Disconnect the sensor and measure heater element resistance with a multimeter; compare to OEM specification (typically 5–20 Ω depending on sensor temperature)
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7
If wiring and fuse test good but resistance is out of spec, replace the oxygen sensor; if all checks pass, suspect PCM heater driver failure
Related powertrain codes
Frequently asked questions
Can I drive with P0042 active?
Yes — the vehicle is generally safe to drive short-term because Sensor 3 is a post-catalyst monitor rather than an active fuel-control sensor. However, the PCM cannot detect catalytic converter degradation while the fault is present, and you will likely fail an emissions test, so the fault should be repaired promptly.
What is Bank 1 Sensor 3, and how is it different from Sensors 1 and 2?
Sensor 1 is upstream of the catalytic converter (used for fuel trim), Sensor 2 is immediately downstream of the main catalyst (monitors catalyst efficiency), and Sensor 3 is downstream of a secondary catalyst or particulate filter on longer exhaust systems. Not all vehicles have a Sensor 3; it is common on trucks with long exhaust runs or on vehicles with multiple catalysts.
Will replacing the oxygen sensor always fix P0042?
In most cases a failed heater element inside the sensor is the cause, so replacement resolves the code. However, always test the fuse and wiring first — if the root cause is a shorted harness, a new sensor will fail again quickly.
Why does the heater element fail?
The heater element is a ceramic resistor that endures extreme thermal cycling. Over time (typically 80,000–150,000 km) the element can crack or break internally. Moisture entering a damaged connector can also corrode the heater terminals and cause an open circuit.
Disabling P0042 in software
RaceTune can permanently disable P0042 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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