Degraded or contaminated direct ozone reduction catalyst, often from prolonged exposure to oil ash or coolant; Faulty ozone reduction catalyst sensor giving a false low-efficiency reading; Damaged or corroded wiring or connector at the ozone reduction catalyst sensor; Exhaust leak upstream of the catalyst allowing unmetered air to skew the efficiency calculation; Failed ozone reduction catalyst monitor, such as a cracked substrate or melted washcoat; Software calibration issue in the powertrain control module misinterpreting sensor data

DTC codes / P25xx · Vehicle speed, idle control and auxiliary inputs / P2577

P2577Direct Ozone Reduction Catalyst Efficiency Below Threshold

Q: What does code P2577 mean?

Direct Ozone Reduction Catalyst Efficiency Below Threshold. P2577 indicates that the powertrain control module has determined the direct ozone reduction catalyst is not converting pollutants efficiently enough, based on signals from its dedicated sensor. The code is set when the catalyst's measured efficiency falls below the calibrated threshold for a sustained period, often due to a degraded catalyst or a faulty sensor circuit.

P2577 indicates that the powertrain control module has determined the direct ozone reduction catalyst is not converting pollutants efficiently enough, based on signals from its dedicated sensor. The code is set when the catalyst's measured efficiency falls below the calibrated threshold for a sustained period, often due to a degraded catalyst or a faulty sensor circuit.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy as the system runs less efficiently
In some cases, a faint sulfur or ammonia odor from the exhaust after driving
Intermittent illumination of the light if the fault is circuit-related rather than a failed catalyst
No drivability complaints in most instances, as the ozone reduction system does not affect engine power

Common causes, most likely first

Degraded or contaminated direct ozone reduction catalyst, often from prolonged exposure to oil ash or coolant
Faulty ozone reduction catalyst sensor giving a false low-efficiency reading
Damaged or corroded wiring or connector at the ozone reduction catalyst sensor
Exhaust leak upstream of the catalyst allowing unmetered air to skew the efficiency calculation
Failed ozone reduction catalyst monitor, such as a cracked substrate or melted washcoat
Software calibration issue in the powertrain control module misinterpreting sensor data

How to diagnose and fix it

Use a scan tool to verify P2577 is current and check for any related pending or history codes
Visually inspect the ozone reduction catalyst sensor and its wiring for damage, corrosion, or loose connections
Check for exhaust leaks between the engine and the catalyst using a smoke test or visual inspection
Measure the ozone reduction catalyst sensor signal with a multimeter and compare to the manufacturer's specification
Perform a catalyst efficiency monitor test with a scan tool to confirm the catalyst's actual conversion ability
If the sensor is faulty, replace it and clear the code to see if it returns
If the catalyst is confirmed degraded, replace the direct ozone reduction catalyst assembly and reset the monitor

What the ozone catalyst code really means

A P2577 is often set by a lazy ozone reduction catalyst sensor rather than a failed catalyst, so always test the sensor before condemning the converter.
Exhaust leaks upstream of the catalyst can dilute the exhaust sample and trigger a false low-efficiency code, so a smoke test is a cheap first step.
The direct ozone reduction system is separate from the main three-way catalyst, so a P2577 does not indicate a problem with the primary converter.
On some vehicles, a weak battery or charging system fault can cause the module to misread sensor data and set P2577 intermittently.
Clearing the code without repairing the root cause will usually result in the code returning after one or two drive cycles once the monitor runs.

Can I keep driving with P2577?

The vehicle remains safe to drive because the ozone reduction system does not affect engine performance or emissions compliance in most jurisdictions, but have the fault diagnosed soon to avoid potential secondary damage. Avoid long-term driving with the check engine light on, as other unrelated faults could be masked.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.