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P codepowertrainMedium severity

P042BCatalyst Temperature Sensor Circuit Range/Performance (Bank 1 Sensor 2)

The downstream catalyst temperature sensor on Bank 1 is producing a signal that is stuck, slow, or inconsistent with expected exhaust temperatures.

Definition

The PCM cross-checks the Bank 1 Sensor 2 catalyst temperature reading against modeled exhaust temperature, the upstream sensor, and ambient after a cold soak. P042B sets when the signal is electrically in range but irrational — frozen during warm-up, responding too slowly, or offset from the other sensors. The MIL comes on, and because diesels use this sensor to control DPF regeneration temperature, an inaccurate reading can cause failed or overheated regenerations and eventual filter damage.

Common causes

  • Drifted or slow-responding sensor element
  • Soot-encrusted sensor tip insulating it from exhaust gas
  • Intermittent high resistance in the connector or splice
  • Sensor not fully seated in its exhaust bung
  • Exhaust leak near the sensor cooling the gas stream locally

Common misdiagnoses

  • DPF or catalyst replaced for a sensor accuracy fault
  • Upstream sensor replaced instead of the Sensor 2 position
  • Connector corrosion overlooked while the sensor is replaced twice

Troubleshooting steps

  1. 1.Run a cold-soak rationality check

    With the vehicle parked overnight, all exhaust temperature sensors plus ECT and IAT should read within about 5 C of each other; flag any outlier.

  2. 2.Graph the sensor during warm-up

    Watch Bank 1 Sensor 2 track the upstream sensor with a modest lag; a flat or wildly delayed trace indicates a lazy element.

  3. 3.Wiggle-test the harness

    Flex the connector and lead while graphing the PID; spikes or dropouts expose intermittent terminals.

  4. 4.Pull and inspect the sensor

    Check the tip for heavy soot or oil coating and verify the sensor threads fully into the boss so the tip sits in the gas stream.

  5. 5.Check for nearby exhaust leaks

    Inspect clamps and joints just upstream of the sensor; leaking joints dilute the exhaust with cool outside air.

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