P0423 means heated catalyst efficiency below threshold bank 1. The ECM saw a value or response outside its calibrated window for the emission control system. Real-world fixes usually start with the highest-probability cause below for your year, make, and model - not a parts-cannon guess.
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Sensor response or heater current is outside the ECM window.
#2Exhaust leak before the sensor$100-$500
0%
False air skews readings and trips efficiency or response codes.
#3Wiring/heater fuse or relay$20-$150
0%
Open heater supply or ground kills warm-up and readiness.
#4Catalytic efficiency or EVAP component$150-$1800
0%
Downstream monitors fail when the cat or EVAP path is compromised.
Symptoms Radar
Check engine light on (or flashing if misfire-related)
Possible rough running or fuel economy drop
Failed emissions readiness or inspection risk
Intermittent stumble under load
Related codes stored with freeze-frame data
Repair Cost
DIY
$40-$280
Shop
$150-$750
Diagnose It
1Confirm P0423 is current (not history-only) and save freeze-frame RPM, load, and fuel trim.
2Inspect connectors and harness for the listed sensor/actuator before replacing parts.
3Command the actuator or graph the sensor live with a scan tool; compare to known-good values.
4Fix the root cause, clear codes, and verify monitors complete on a mixed drive cycle.
FAQ
Can I drive with a P0423 code?
Usually drivable short-term, but fix soon to avoid catalyst damage or a roadside no-start.
How much does it cost to fix P0423?
DIY parts typically land around $40-$280. A shop diagnosis plus repair commonly runs $150-$750, depending on whether it is a sensor, harness, or major component.
What is the most common cause of P0423?
Failed O2/HO2S or heater circuit is the most common starting point (~36%), followed by exhaust leak before the sensor.