OBD-II trouble code
P0438: Catalyst Temperature Sensor High Input (Bank 2)
The bank 2 catalyst temperature signal is above the ceiling the chart allows. On an NTC pull-up circuit that high reading is an open. Ask what was unplugged on this bank, then prove the open with a measurement. The connector location comes from the diagram.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $650
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0438 mean?
On an NTC thermistor with a pull-up, a high signal is the signature of an open. Confirm that sensor type from the diagram before this test is used. A thermocouple does not read an open the same way. With the sensor connected, the module's pull-up voltage is divided down by the sensor's resistance and the result is a sensible number. Break the circuit anywhere — the element, a terminal, the wire — and nothing pulls that voltage down, so it rises to the top of the scale. The module then holds a value corresponding to a converter it believes is stone cold, and P0438 is the result. That is why an open reads as maximum voltage and not as zero, and it is worth understanding because it drives the whole test sequence: an open is a resistance question, so it is answered with measurements, not with a visual hunt for burnt insulation.
Ask what was unplugged on bank 2 recently, then prove it with a measurement. On some transverse engines the bank 2 connector is behind the intake and is disturbed by intake, plug, or manifold work. That is a layout, not a share of cases, and bank 1 is not assumed to sit in the open. An unlatched connector, a backed-out terminal, or a bent pin is one open the resistance test will find. It is not ranked ahead of an open element.
The more ordinary causes remain: a sensor element that has failed open after years of thermal cycling, corrosion that has eaten a terminal through, or a broken conductor inside insulation that still looks perfect from the outside. That last one is worth naming because it defeats visual inspection entirely and only a continuity measurement will find it.
The consequence of leaving it is not felt on the road, which is the reason this code gets postponed and then becomes urgent at an inconvenient moment. The open does not by itself change how the engine runs, and a fuel-economy change is not what this circuit code describes. The bank 2 catalyst monitor will not complete while its temperature signal is implausible. An inspection that requires that monitor will not pass on the strength of bank 1 alone.
Common causes
- Connector at the bank 2 sensor not fully latched after recent work on that bank
- Terminal pushed back or bent in the connector during blind reassembly behind the intake manifold
- Sensor element failed open internally after prolonged thermal cycling
- Signal or ground wire broken inside insulation that still looks undamaged from outside
- Corrosion having eaten through a terminal at a low, exposed connector
- Harness cut, pinched or pulled apart during intake, exhaust or spark plug work on bank 2
- Loss of the sensor ground circuit, which produces the same high reading as a broken signal wire
- Module input circuit failure, which is uncommon and belongs at the end of the list
Symptoms
- Check engine light with P0438 stored
- Bank 2 catalyst temperature reading at the top of the scale, or showing as cold no matter how long the engine has run
- Bank 1 temperature rising normally on the same screen while bank 2 does not move
- Catalyst monitor not completing, so an emissions test cannot be passed
- No noticeable change in how the vehicle drives
- Noticed after unrelated work on that bank is a reason to inspect the connector that job moved
Diagnostic steps
- 1.Ask what work has been done recently. Rear-bank spark plugs, an intake gasket, an exhaust manifold or a converter all put hands on this connector, and a disturbed connection is a leading cause of this code.
- 2.Inspect the bank 2 connector before anything else. Check that it is fully latched, that no terminal has been pushed back in the body, and that no pin is bent.
- 3.Compare the two banks' live catalyst temperature values. Bank 1 climbing normally while bank 2 sits pinned at maximum is the classic open-circuit pattern.
- 4.Jumper the sensor's signal and ground terminals together at the harness connector with the key on. If the reading drops to the bottom of the scale, the wiring and module input are proven good and the sensor is the open.
- 5.If the reading stays high with that jumper fitted, the open is in the harness or the ground, not the sensor, and the measurement moves back toward the module.
- 6.Measure sensor resistance at ambient. An infinite reading confirms an internally open element.
- 7.Check continuity on both the signal and the ground circuits from the sensor connector to the module connector, since a lost ground produces an identical high reading.
- 8.Flex and wiggle the harness with the reading live, concentrating on any section that was disturbed during recent work.
- 9.Do not condemn the converter on this code. Bank 2 conversion performance is judged by P0430 and its siblings, not by this one.
Repair cost
$60 – $650
Read the printed bracket as a planning span, not a quoted price. The low end names a connector that was left unlatched or a terminal repaired after the resistance test finds it. The high end names the sensor, or a harness repair, after that test has passed the connector. An inspection that requires the bank 2 monitor will not pass while the signal stays implausible.
Estimate your repair
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Open the Repair Cost Estimator with wiring harness / circuit repair preselected. Adjust labor rate and vehicle category to fit your situation.
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.