P0438
Catalyst Temperature Sensor Circuit High (Bank 2, Sensor 1)P0438 is a generic OBD-II powertrain diagnostic trouble code: Catalyst Temperature Sensor Circuit High (Bank 2, Sensor 1). It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0438 means
P0438 — Catalyst Temperature Sensor Circuit High Input (Bank 2, Sensor 1) — is the electrical complement to P0437 and the Bank 2 mirror of P0428. The PCM has detected a signal voltage at the Bank 2 catalyst temperature sensor input that is at or above the maximum threshold for a valid reading. The indicated temperature is therefore impossibly high or pegged at the top of the measurement scale.
On NTC thermistor sensor circuits the most common cause of a high-circuit reading is an open circuit: with the signal wire broken or the sensor element internally open, the PCM's internal pull-up resistor (if fitted) pulls the input rail to the reference voltage, which the PCM interprets as maximum (or near-maximum) sensor resistance — corresponding to an impossibly cold or impossibly high temperature depending on the circuit topology. On thermocouple-based sensors an open conductor produces a saturated or out-of-range output at the amplifier stage that reads as an over-range high value. The practical effect in both cases is complete loss of Bank 2 catalyst temperature feedback.
On gasoline platforms this primarily means the MIL illuminates and Bank 2 catalyst monitoring is suspended. On diesel vehicles where the Bank 2 exhaust temperature sensor participates in DPF thermal management, loss of this signal can degrade regen control quality. Repair approach is the same as for P0427 but with the harness inspection focused on open-circuit failure modes rather than shorts to ground.
Common causes
Most-frequently reported root causes when P0438 is logged.
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1
Open circuit in the Bank 2 catalyst temperature sensor signal wire between the connector and the PCM.
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2
Internal open in the sensor element (broken NTC thermistor bead, broken thermocouple wire, or thermally degraded sensor that has gone open-circuit).
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3
Corroded or backed-out pin at the sensor harness connector leaving the PCM input floating.
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4
Open in the sensor ground or reference return wire causing the signal rail to be pulled high by the PCM's internal pull-up resistor.
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5
Heat-damaged wiring harness with a broken conductor — common near exhaust routing and heat shields on Bank 2.
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6
Sensor failure from thermal overstress (e.g., excessive exhaust temperatures from a lean condition or prior DPF runaway on diesel platforms).
Symptoms drivers notice
How to diagnose P0438
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and verify the Cat Temp B2S1 PID is pegged high or reads above the physically possible range; log all companion DTCs.
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2
With KOEO, measure signal voltage at the sensor harness connector with the sensor disconnected — a voltage near the PCM reference (typically 5 V) is normal for an NTC circuit with a pull-up; if the sensor is connected and still pegged high, the sensor element is likely open internally.
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3
Disconnect the sensor and measure resistance across the sensor terminals — infinite (OL) reading confirms an open sensor element.
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4
Inspect the wiring harness from the sensor connector to the PCM for breaks, burned insulation, or mechanical damage caused by proximity to the exhaust.
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5
Check for backed-out or corroded pins at the sensor connector by measuring continuity from each connector pin to the corresponding PCM cavity.
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6
After confirming harness integrity, replace the sensor if an internal open is confirmed.
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7
Clear codes, verify with live data that the PID reads plausibly and tracks engine warm-up correctly after repair.
Related powertrain codes
- P0400 — Exhaust Gas Recirculation Flow Malfunction
- P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- P0403 — Exhaust Gas Recirculation Circuit Malfunction
- P0404 — Exhaust Gas Recirculation Circuit Range/Performance
- P0405 — Exhaust Gas Recirculation Sensor A Circuit Low
Frequently asked questions
Why does an open circuit read as a high signal rather than zero?
If the PCM has an internal pull-up resistor on the sensor input pin, a broken wire or open sensor element leaves the input rail pulled toward the reference voltage, which the PCM reads as the high end of the sensor range. The exact behavior depends on the circuit design — some platforms read open as high, others as erratic or defaulting to a fixed high value.
Is P0438 the Bank 2 equivalent of P0428?
Yes. P0438 mirrors P0428 exactly — the fault mechanism, diagnostic steps, and likely causes are the same; only the physical location of the Bank 2 sensor and its dedicated wiring run differ.
Could extreme exhaust temperatures burn out the sensor and cause P0438?
Yes. A lean misfire, runaway DPF regen on diesel platforms, or sustained high-load operation can expose the sensor to temperatures beyond its rated range, destroying the sensing element and leaving it open-circuit. If P0438 appears alongside lean mixture codes or DPF-related faults, inspect the sensor for physical heat damage before assuming a simple wiring fault.
Disabling P0438 in software
RaceTune can permanently disable P0438 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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