Catalyst Systems | TuningBot

Key takeaways

The three-way catalyst needs stoichiometric oscillation to clean all three pollutants at once.
Light-off temperature is everything: cold catalysts convert nothing.
Sustained rich running or misfires overheat and destroy the substrate.

Catalytic converters use precious metal catalysts to convert harmful exhaust gases (CO, HC, NOx) into less harmful substances (CO2, H2O, N2). The ECU manages catalyst temperature and monitors conversion efficiency.

Overview

Three-way catalysts (gasoline) require precise stoichiometric AFR. Diesel oxidation catalysts (DOC) handle CO and HC, while SCR handles NOx. Catalyst light-off temperature must be reached quickly after cold start.

Controlled Signals

Pre-catalyst lambda sensor
Post-catalyst lambda sensor
Catalyst temperature (measured or modeled)
Catalyst heating strategies

Maps Involved

Catalyst Heating Maps (retarded timing, secondary air)
Light-off Temperature Targets
Efficiency Monitoring Thresholds
Lambda Control Maps

Logic Sequence

Cold Start → Catalyst Heating Strategy
         ↓
Monitor Catalyst Temperature Rise
         ↓
Light-off Achieved → Normal Lambda Control
         ↓
Continuous Efficiency Monitoring

Calibration Objectives

Fast catalyst light-off
Maintain high conversion efficiency
Prevent catalyst overheating

Calibration Strategy

Catalyst delete requires O2 sensor simulation or deactivation
Sports catalysts may require lambda sensor offset
Verify AFR control after any exhaust modifications

Diagnostics

P0420/P0430 catalyst efficiency below threshold
Oxygen sensor response faults
Catalyst overtemperature

Best Practices

Catalyst removal is illegal for road use
Rich running damages catalysts — fix fueling issues promptly
High-flow sports cats maintain emissions compliance with better flow

In Calibration Practice

The efficiency monitor compares oxygen storage behavior across the catalyst, which is why downstream lambda activity — not just codes — tells you catalyst health. Calibration touches it indirectly: enrichment strategy, misfire margins and heating strategies all decide whether the substrate lives; sustained misfire is the fastest catalyst killer a file can permit.

What kills catalytic converters?

Misfires above all — raw mixture burning inside the brick melts it. Chronic oil or coolant consumption poisons the coating; sustained overtemperature from aggressive maps sinters it. Most ‘catalyst faults’ trace back to an engine problem upstream.

Why does my car run rich at full load — is the catalyst involved?

Yes: full-load enrichment is largely component protection, cooling exhaust gas to keep catalyst and turbine below their limits. Leaning that region out for power without EGT awareness is a classic way to shorten both components’ lives.

Related: on supported vehicles this system is directly addressed by our Lambda/O2 removal service. See also Oxygen Sensors (Lambda) in the knowledge base.