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P codepowertrainMedium severity

P042ACatalyst Temperature Sensor Circuit (Bank 1 Sensor 2)

There is an electrical fault in the circuit for the second (downstream) catalyst temperature sensor on Bank 1.

Definition

Sensor 2 is the mid-bed or outlet catalyst temperature sensor on Bank 1; on diesel applications these exhaust gas temperature sensors along the DOC and DPF are critical for managing regeneration. P042A is the general circuit-malfunction code for this sensor, set when the signal opens, shorts, or leaves its valid electrical range. The MIL illuminates, and on diesels the PCM may inhibit active DPF regeneration, which can eventually lead to filter soot loading and reduced-power operation if left unrepaired.

Common causes

  • Failed sensor element from repeated heat cycling (common at high mileage)
  • Heat-damaged or chafed wiring along the exhaust tunnel
  • Corroded or moisture-contaminated connector
  • Harness damage from prior exhaust or DPF service
  • ECM input circuit fault (rare)

Common misdiagnoses

  • DPF or catalytic converter replaced for a simple sensor circuit fault
  • Wrong temperature sensor replaced because sensor positions were confused
  • Oxygen sensor replaced instead of the temperature sensor

Troubleshooting steps

  1. 1.Identify the correct sensor position

    Consult the service information to locate Bank 1 Sensor 2 — many exhausts carry three or four temperature sensors and they are easily confused.

  2. 2.Inspect the wiring and connector

    Trace the lead for melted insulation, chafe-throughs, and green corrosion inside the connector, especially near heat shields.

  3. 3.Measure sensor resistance

    Compare the reading at a known temperature to spec; catalyst/EGT sensors range from a couple hundred ohms to several kilohms depending on type, and OL means the element is open.

  4. 4.Verify reference voltage

    With the connector unplugged and key on, confirm the supply (typically 5 volts) from the ECM is present at the harness.

  5. 5.Compare all exhaust temperature PIDs cold

    After an overnight soak, every exhaust temperature sensor should agree with ambient within a few degrees; the outlier is the faulty circuit.

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