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P codepowertrainMedium severity

P0427Catalyst Temperature Sensor Low Input (Bank 1)

The signal from the Bank 1 catalyst temperature sensor has dropped below its normal operating voltage range.

Definition

The PCM monitors the Bank 1 catalyst temperature sensor circuit and sets P0427 when the signal voltage falls below the minimum plausible threshold, which usually indicates a short to ground or an internally shorted sensor. Depending on sensor design the pegged signal translates to an impossibly high or low temperature, so the PCM discards the reading and suspends catalyst temperature monitoring. The engine runs normally, but the MIL is on and the vehicle will fail emissions testing; on diesels, aftertreatment regeneration control may be affected.

Common causes

  • Signal wire chafed through and shorted to ground on the body or exhaust
  • Internally shorted catalyst temperature sensor
  • Moisture or corrosion bridging terminals inside the connector
  • Harness crushed or pinched during prior exhaust repairs
  • ECM input circuit fault (rare)

Common misdiagnoses

  • Catalytic converter replaced for an electrical circuit-low fault
  • Sensor replaced when the harness short to ground is the actual cause
  • ECM replaced before the wiring is checked

Troubleshooting steps

  1. 1.Read the temperature PID

    A value pegged at the bottom or top of its range with the engine warm confirms an electrical fault rather than a real temperature problem.

  2. 2.Unplug the sensor and re-check the PID

    If the reading flips to the open-circuit default value, the short is in the sensor; if it stays pegged, the fault is in the harness or ECM side.

  3. 3.Test the signal wire for a short to ground

    With both the sensor and ECM connectors unplugged, resistance from the signal wire to chassis ground should be out of range (OL).

  4. 4.Inspect the harness routing

    Trace the lead along the exhaust and floor pan for chafe points, melted sections, or damage from previous undercar work.

  5. 5.Measure sensor resistance

    Compare the element resistance at a known temperature against the specification chart; a near-zero reading indicates an internal short.

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