Emissions

P0042 — O2 Sensor Heater Control Circuit, Bank 1 Sensor 3

Severity: low Safe to drive

What P0042 means

The engine control module monitors the heater circuit of the Bank 1, Sensor 3 oxygen sensor. The heater brings the sensor up to operating temperature quickly so it can read exhaust content accurately. P0042 sets when heater current or resistance falls outside the expected range, due to an open, a short or a failed heater element. Sensor 3 is typically downstream of the main catalyst, but confirm the layout for your engine.

Symptoms

  • Check engine light on
  • Emissions readiness monitors not completing
  • Possible rough running on cold starts
  • Possible slight increase in fuel consumption
  • Occasionally unstable idle

Possible causes

  1. Open or shorted wiring in the sensor heater circuit
  2. Corroded or damaged sensor connector
  3. Failed heater element in the Bank 1 Sensor 3 sensor
  4. Blown fuse or power supply problem for the heater circuit
  5. Faulty heater driver inside the ECM (uncommon)

How to diagnose

  1. Scan all codes and freeze frame data, clear the code and see if it returns
  2. Confirm which sensor is Bank 1 Sensor 3 on your engine
  3. Inspect the harness and connector for cracks, melting, corrosion or rubbing on the exhaust
  4. Check the heater circuit fuse and relay using the wiring diagram
  5. Measure the sensor heater resistance and compare it to the manufacturer spec
  6. Check for power and the control signal at the connector with the ignition on
  7. Test the wires back to the ECM for opens and shorts to ground or power
  8. If wiring and sensor test good, evaluate the ECM driver

Common fixes

  • Repair the damaged wiring or replace the connector
  • Replace the Bank 1 Sensor 3 oxygen sensor
  • Replace the fuse or relay after finding the cause of the overload
  • Replace or reprogram the ECM if a driver fault is confirmed

P0042 on different brands

Ford · manuals
On Ford, the extended code P0042:1E in the 2024-2025 monitor strategy refers to HO2S heater resistance, Bank 1; it identifies the detected condition, not a confirmed failed part.

Questions and answers

Can I drive with a P0042 code?

Usually yes, as it does not directly threaten the engine. Still, don't put it off long, since the light stays on and you may fail an emissions test.

What most often causes P0042?

Most often it is damaged heater wiring or a bad connector, or an oxygen sensor with a failed heater. ECM failure is rare.

Should I replace the sensor right away?

No. First check the fuse, wiring and heater resistance. Replace the sensor only if its heater is out of spec.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.