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DTC codes / P04xx · Auxiliary emission controls / P0428

P0428Catalyst Temperature Sensor High (Bank 1)

P0428 is a generic OBD-II code that indicates the powertrain control module (PCM) has detected an abnormally high signal voltage from the catalyst temperature sensor on bank 1, which monitors the temperature of the catalytic converter. The PCM interprets this high voltage as an excessively hot catalyst, which can lead to converter damage and increased emissions.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Reduced engine performance or power, especially under load
  • Poor fuel economy due to the PCM entering a protective mode
  • Possible overheating smell or visible heat damage around the catalytic converter
  • In rare cases, the engine may enter limp mode to prevent damage

Common causes, most likely first

  1. Faulty catalyst temperature sensor (bank 1) sending a high voltage signal
  2. Damaged or corroded wiring or connector at the sensor or PCM
  3. Exhaust restriction causing the catalytic converter to overheat
  4. Engine running too rich or too lean, leading to elevated exhaust temperatures
  5. Failed catalytic converter substrate causing excessive heat
  6. PCM internal fault misinterpreting the sensor signal

How to diagnose and fix it

  1. Use an OBD-II scanner to verify the code and check live data for the catalyst temperature sensor voltage
  2. Inspect the sensor wiring and connector for damage, corrosion, or loose pins
  3. Test the sensor resistance and voltage output according to the manufacturer's specifications
  4. Check for exhaust restrictions (e.g., clogged converter) using a backpressure gauge
  5. Diagnose engine performance issues (fuel trims, misfires) that could cause overheating
  6. Replace the catalyst temperature sensor if faulty
  7. Clear the code and perform a road test to confirm the repair

Catalyst temperature sensor high voltage pitfalls

  • A high voltage signal from the catalyst temperature sensor often indicates a sensor or wiring fault rather than an actual overheating converter, so always test the circuit before condemning the converter.
  • Exhaust leaks upstream of the sensor can introduce oxygen and cause the sensor to read artificially high temperatures, leading to misdiagnosis.
  • The catalyst temperature sensor is sometimes confused with the oxygen sensor; ensure you are testing the correct component for bank 1.
  • A failing catalytic converter can cause the sensor to read high, but replacing the converter without addressing the root cause (such as rich fuel mixture) will lead to repeat failure.
  • On some vehicles, the catalyst temperature sensor is part of a combined sensor assembly, and a fault in the reference voltage circuit can set this code.

Can I keep driving with P0428?

It is generally safe to drive with P0428 set, but avoid heavy loads and prolonged high-speed driving to prevent further catalyst damage. Have the vehicle diagnosed promptly, as continued driving with an excessively hot catalyst can lead to costly repairs.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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