P0428–P0438 Catalyst Temperature Sensor Circuit High Bank 1 Sensor 1
The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0428 | Catalyst Temperature Sensor Circuit High Bank 1 Sensor 1 | Investigate soon |
P042D | Catalyst Temperature Sensor Circuit High Bank 1 Sensor 2 | Investigate soon |
P0438 | Catalyst Temperature Sensor Circuit High Bank 2 Sensor 1 | Investigate soon |
The definition
P0428: Catalyst Temperature Sensor Circuit High Bank 1 Sensor 1.
P0428 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Unplug the sensor and check whether the signal falls. If it stays high, suspect the wiring rather than the sensor.
- Check the earth path - a missing earth makes a sensor read high.
What actually causes this on a VAG
This is a temperature sensor sitting in the exhaust in the front position of the catalyst system - "Bank 1 Sensor 1" is the first one in that path. The ECU uses exhaust temperature to protect expensive hardware: the turbocharger and the catalyst, and on cars fitted with a particulate filter it is also part of how regeneration is managed. "Circuit High" means the voltage on the signal line is above the range the ECU will accept, which in practice means a broken wire, a disconnected or corroded plug, or a failed sensor element rather than a real exhaust temperature. One thing to do before anything else: VAG fits several different exhaust temperature sensors, each with its own set of fault codes, so read the fault in a VAG-aware tool and note which component it actually names.
Most likely causes, in order
- A broken sensor lead or corroded connector. The thin lead runs along the exhaust and is clipped to a heat shield or the underbody, so it lives in heat, spray and salt. Snapped leads, insulation melted against the exhaust, and green corrosion inside the plug are all common. Follow the wire from the sensor tip to its plug and inspect every inch - this is the most likely cause by a wide margin and it costs nothing to look.
- Disturbed during previous exhaust, turbo or filter work. If the fault appeared straight after a job in that area, suspect a plug left unclipped, a lead trapped or pinched, or a sensor not screwed fully home. Check what was touched before you buy anything.
- The sensor has failed internally. These do wear out; they are heat-cycled hard for their whole life. Judge it on live data rather than on the code text: a sensor with a failed element sits pinned at one end of its scale and does not move, while a working one climbs as the engine warms and rises further under load. If the lead and plug are genuinely sound and the reading never tracks the exhaust, the sensor is the part to change.
- Wiring damage further up towards the ECU. A break or a corroded multiplug between the sensor and the control module. Uncommon compared with damage down at the sensor end, so leave it until the visible wiring has been cleared.
Worth knowing: Do not simply clear this and carry on, particularly on a car with a particulate filter. The ECU leans on exhaust temperature both to protect the turbo and catalyst and to decide when it can safely run a regeneration, and a signal it does not trust can mean regenerations are blocked or abandoned while the filter quietly loads with soot. A cheap sensor sorted now is a great deal better than a forced regeneration or a new filter later. Expect the sensor thread to be seized solid into the exhaust after years of heat cycling, and be prepared for it to shear. Note also that these sensors are normally sold complete with their lead and plug - the thin lead cannot be reliably spliced or repaired.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does P0428 mean?
P0428 means Catalyst Temperature Sensor Circuit High Bank 1 Sensor 1. The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can I drive with P0428?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.