Catalyst Efficiency Monitor

Non-Continuous

Requires 5-10 minutes of steady highway cruising at 40-60 mph after the engine and catalyst are fully warmed up. Total drive time including warm-up: 15-25 minutes.

What This Monitor Checks

The Catalyst Efficiency Monitor evaluates the oxygen storage capacity of the three-way catalytic converter by comparing the switching frequency and amplitude of the upstream (pre-cat) O2 sensor to the downstream (post-cat) O2 sensor. A healthy catalyst absorbs and releases oxygen, which dampens the downstream sensor signal; as the catalyst degrades, the downstream sensor begins to mirror the upstream sensor's rapid switching pattern. The PCM calculates a catalyst efficiency ratio and compares it against a calibrated threshold.

Why It Matters for Emissions

The catalytic converter is the single most important emissions reduction component, converting 90-98% of HC, CO, and NOx into harmless CO2, H2O, and N2. A degraded catalyst can cause tailpipe emissions to exceed federal standards by 5-20x, making this monitor critical for emissions compliance.

Drive Cycle Steps

Start with a fully warmed-up engine (coolant above 180°F / 82°C) and ensure the vehicle has been driven long enough for the catalyst to reach operating temperature (typically 600-1,000°F internally).

Drive at a steady speed between 40-60 mph on a flat road for 4-6 minutes with minimal throttle variation — the PCM needs stable closed-loop fueling to compare upstream and downstream O2 signals.

Avoid hard acceleration, braking, or gear changes during the steady cruise phase; the test aborts if fuel trim corrections exceed a threshold.

Some vehicles require a brief deceleration (coast-down from 55 to 40 mph in gear with throttle closed) to trigger the oxygen storage evaluation.

If the test doesn't complete on the first attempt, repeat the steady cruise after a 2-minute moderate driving interval.

Prerequisites

Engine coolant temperature must be above 180°F (82°C) and stable — not still climbing.
Upstream and downstream O2 sensors must be fully operational with no pending or confirmed O2 sensor DTCs.
Fuel trim values (STFT + LTFT) must be within ±10% to ensure accurate catalyst evaluation during steady-state.
The vehicle must be driven at least 5-10 minutes prior to the test to ensure the catalyst substrate has reached light-off temperature.

Common Failure Reasons

Catalyst substrate degradation from age, high mileage, or previous misfire/rich-running damage reducing oxygen storage capacity below the PCM threshold.
Upstream O2 sensor sluggishness (slow response time) causing the PCM to misinterpret catalyst efficiency calculations.
Exhaust leaks between the upstream O2 sensor and the catalyst introducing ambient air that skews efficiency readings.
Engine oil consumption (from worn valve seals or piston rings) contaminating the catalyst washcoat with phosphorus and zinc from the oil.
Use of leaded fuel or silicone-contaminated RTV sealant poisoning the catalyst surface.

Pro Tips

Use Mode $06 test ID for catalyst monitoring to see how close the converter's efficiency ratio is to the failure threshold — a value approaching the limit means replacement is likely needed soon even if the monitor currently passes.
On V-engine vehicles with dual catalysts, Bank 1 and Bank 2 catalysts are tested independently — one bank can fail while the other passes.
A marginal catalyst may pass in summer but fail in winter because the lower ambient temperature increases the time to reach catalyst light-off temperature, reducing test window availability.
If replacing the catalyst, always diagnose and repair the root cause of failure first (misfires, rich running, oil consumption) — otherwise the new converter will fail prematurely.

Frequently Asked Questions

What does the Catalyst Efficiency Monitor check?

How do I get the Catalyst Efficiency Monitor to set ready?

Follow the drive cycle: Start with a fully warmed-up engine (coolant above 180°F / 82°C) and ensure the vehicle has been driven long enough for the catalyst to reach operating temperature (typically 600-1,000°F internally). Drive at a steady speed between 40-60 mph on a flat road for 4-6 minutes with minimal throttle variation — the PCM needs stable closed-loop fueling to compare upstream and downstream O2 signals. Avoid hard acceleration, braking, or gear changes during the steady cruise phase; the test aborts if fuel trim corrections exceed a threshold. Some vehicles require a brief deceleration (coast-down from 55 to 40 mph in gear with throttle closed) to trigger the oxygen storage evaluation. If the test doesn't complete on the first attempt, repeat the steady cruise after a 2-minute moderate driving interval. Estimated completion: Requires 5-10 minutes of steady highway cruising at 40-60 mph after the engine and catalyst are fully warmed up. Total drive time including warm-up: 15-25 minutes.

Why does the Catalyst Efficiency Monitor keep failing?

Common failure reasons include: Catalyst substrate degradation from age, high mileage, or previous misfire/rich-running damage reducing oxygen storage capacity below the PCM threshold.; Upstream O2 sensor sluggishness (slow response time) causing the PCM to misinterpret catalyst efficiency calculations.; Exhaust leaks between the upstream O2 sensor and the catalyst introducing ambient air that skews efficiency readings..

OBDHut Mobile App

Scan codes directly from your car with the OBDHut app.

Coming Soon

Quick Info

Type

Non-Continuous

Completion

Requires 5-10 minutes of steady highway cruising at 40-60 mph after the engine and catalyst are fully warmed up. Total drive time including warm-up: 15-25 minutes.

Resets on Clear

Yes