OBD-II trouble code
P0426: Catalyst Temperature Sensor Range/Performance (Bank 1)
The catalyst temperature sensor on bank 1 is reporting a number that is electrically perfectly respectable and physically wrong. Nothing is open, nothing is shorted, and the value is simply not what the rest of the vehicle says the temperature ought to be.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0426 mean?
Range and performance codes are the awkward middle child of every sensor family, and on a temperature sensor the awkwardness has a particular shape. There is no dramatic electrical failure to find. The circuit passes every continuity and short-to-ground test you can throw at it. The sensor sends a voltage that sits comfortably within its normal working window. The module rejects it anyway, because it has cross-checked that number against what it knows about engine load, airflow, run time and its own thermal model of the converter, and the two accounts do not agree.
The cold-soak comparison is the test that belongs at the top of this page. After a cold soak, compare catalyst temperature with the other temperature parameters and use the difference the service information allows. Do not use a fixed number of degrees. A sensor that matches the others cold and then lags on a warm-up is a response check, which is what a range/performance monitor can be looking at. The allowed rate is the calibration.
A coated tip is one thing to inspect, because a layer of carbon or ash can slow the reading without opening the circuit. It is not a ranking of causes. P0427 and P0428 are the low and high circuit codes. A reading that stays inside the voltage window and still fails the performance test is this code. The pass value is the chart.
There is a second cause with a completely different fix, and it is one that punishes assumption. If the sensor has been replaced with a part of the wrong specification, or fitted in the wrong position on the exhaust, it will read a real temperature accurately — just not the temperature the module expects at that point in the system. Moving a probe six inches further from the converter changes the number it reports without changing anything about the sensor's health. When this code appears after the sensor was moved, compare the new probe's position with the one the diagram shows. A probe moved along the pipe reports a different temperature without a wiring fault.
A range fault on this sensor does not by itself describe a change in how the engine runs. What it will not do is complete its catalyst-related monitors reliably, and a module working from a temperature it does not trust may decline to run other diagnostics that depend on it.
Common causes
- Sensor tip coated with carbon, soot or oil ash, so it reads correctly at steady state but responds too slowly to changes
- Sensor drifted out of calibration with age, producing a consistent offset from true temperature
- Incorrect or aftermarket sensor fitted with a different resistance curve to the original
- Sensor installed in the wrong position on the exhaust after previous work, reading a genuine but unexpected temperature
- High resistance in the connector or harness adding a small voltage error without breaking the circuit
- Exhaust leak near the probe letting ambient air reach it and depress the reading
- Genuine thermal fault at the converter that the module is interpreting as an implausible sensor reading
- Coolant intrusion into the exhaust from an internal engine fault, which changes the thermal behaviour the sensor sees
Symptoms
- Check engine light with P0426 stored and no other electrical faults present
- Catalyst temperature reading that looks believable but does not track the other temperature sensors
- Reading that lags visibly behind engine load changes during a road test
- Emissions readiness monitors that will not complete
- No fuel-control change from this sensor reading
- Code returning after a sensor replacement, if the replacement part was the wrong specification
Diagnostic steps
- 1.After a cold soak, compare catalyst temperature with coolant and intake air and judge the gap against the service information. There is no universal degree limit on this page.
- 2.If the cold reading is correct, the fault is a response-rate problem rather than an offset. Log the sensor through a full warm-up and a hard acceleration and watch whether it lags the change rather than missing the value.
- 3.Ask what work has been done to the exhaust recently. A sensor in the wrong position, or a non-original part with a different curve, will produce this code with nothing physically wrong.
- 4.Remove the sensor and look at the tip. Heavy carbon or ash coating explains a slow response directly, and coolant residue points at a much larger problem inside the engine.
- 5.Check the connector for resistance that the chart calls high. An open-circuit test does not see a connection that is still touching. The ohm limit is the chart, not a fixed few ohms.
- 6.Inspect for exhaust leaks close to the probe. Ambient air reaching the tip depresses the reading in a way that looks exactly like sensor drift.
- 7.Walker Exhaust's infrared screen is a surface temperature on the weld rings, beginning near 350°F and fully lit near 500°F, and it is not this code's threshold. Use the bank 2 sensor as a comparison only when it is the same part and the service information treats the two as equal after a cold soak.
Repair cost
$120 – $600
The low end of the printed bracket is the connector, after the cold-soak comparison in the service information has passed and the terminals have not. The high end is the sensor, the part number the diagram lists, after that comparison fails. Read the bracket as a planning span, not a quoted price. A sensor with a different resistance curve can store this code again.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with exhaust gas temperature sensor replacement 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.