OBD-II trouble code
P0428: Catalyst Temperature Sensor High Input (Bank 1)
The signal voltage from the bank 1 catalyst temperature sensor has risen above its allowed ceiling. On an NTC thermistor that high voltage is a low temperature reading. Confirm the sensor type in the service information, and the cost of that belief is not felt on the road — it is felt at the emissions station.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $550
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0428 mean?
This is the mirror of the low input code and the mirroring is worth being precise about. The module reads the voltage where the sensor and its internal pull-up resistor meet. On a negative temperature coefficient thermistor, resistance rises as the sensor cools, and rising resistance pulls that junction voltage up towards the reference. High voltage therefore reports low temperature. When the circuit opens completely, resistance becomes effectively infinite, the junction sits at full reference voltage, and the module sees the cold end of that sensor's scale. That end is the open-circuit reading, not a measured exhaust temperature.
An open circuit leaves a pull-up at the reference voltage, so an NTC input reads at the cold end of its scale. The connector is one place an open hides, because the terminals are only pressed together and they live on the exhaust. Cleaning and re-tensioning a terminal is the repair when the open is in that connector. It is not a share of cases. The low-input code is a short toward ground, which is a different measurement.
A protection strategy that uses this temperature is in the service information. A false cold reading does not trigger an overheat strategy. The catalyst monitor still needs a plausible temperature before it completes, so an inspection that requires that monitor will not pass while the signal stays open.
One genuine possibility should not be dismissed too quickly. If the vehicle has just been started on a very cold morning, a low reading is correct. The module allows for that. A code sets when the reading stays at the extreme after conditions that should have moved it, so check the reading against run time before assuming the circuit is at fault.
Common causes
- Open circuit at the sensor connector caused by relaxed terminal tension after years of thermal cycling
- Oxidised or corroded terminals making an intermittent then permanent open connection
- Sensor element open, confirmed when bridging the connector swings the reading and the wiring is intact
- Broken conductor inside the insulation where the lead flexes near the sensor body
- Signal or ground circuit open somewhere between the connector and the module
- Connector not fully seated or the locking tab not engaged after previous exhaust work
- Poor ground for the sensor circuit, which raises the measured voltage without a full open
- Module input, checked after the connector bridge shows the harness does not swing the reading
Symptoms
- Check engine light with P0428 stored
- Catalyst temperature pegged at the minimum of the scale even after a long drive, if the sensor is a thermistor type
- Emissions readiness monitors that never complete, so the vehicle cannot be inspected
- No noticeable change in how the vehicle drives or in fuel economy
- Reading that flickers between a real value and the extreme when the harness is disturbed
- Fault appearing shortly after exhaust, converter or oxygen sensor work
Diagnostic steps
- 1.Confirm the sensor technology first. On a thermistor sensor a high input code reports a cold converter, and on a thermocouple design the relationship is reversed. Reading the number the wrong way round will send you looking for the wrong fault.
- 2.Compare the live value with the run time in the service information. A cold reading immediately after start can be correct. A reading that never leaves the cold end of the scale after the warm-up the chart allows is not.
- 3.Go to the connector before the wire. Unlatch it, inspect both halves under good light for green or white corrosion, and check the female terminals for relaxed spring tension by feel with a matching pin.
- 4.Bridge the sensor connector's signal and ground terminals with a jumper and watch the live data. If the reading swings to the opposite extreme, the harness and module are proven good and the sensor is the fault.
- 5.If the reading does not move with the connector bridged, the open is in the harness or at the module. Check continuity on each wire back to the module connector.
- 6.Check the sensor's ground circuit specifically. A high resistance ground raises the measured signal voltage and produces the same code as a partial open on the signal side.
- 7.Wiggle-test the connector and the first few inches of lead with live data running. Intermittent opens from thermal fatigue reveal themselves this way and are otherwise very hard to catch.
- 8.Clean and re-tension the terminals as a first repair where they are the fault. On a thermally cycled connector this is a real fix, not a temporary one, provided the terminals are not corroded through.
Repair cost
$60 – $550
The low end of the printed bracket is the connector, when bridging past it swings the reading. The high end is the sensor, when the bridge shows the harness is intact and the element is open. Read the bracket as a planning span, not a quoted price. The catalyst monitor will not complete while the signal stays at the open-circuit reading.
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DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.