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Heated Catalyst Monitor

Non-Continuous

Requires a cold start with 5-8 minutes of gentle driving. The cold soak requirement means this monitor can typically only be tested once per day.

What This Monitor Checks

The Heated Catalyst Monitor evaluates the performance of catalytic converters equipped with electric heating elements designed to bring the catalyst to light-off temperature more quickly during cold starts. The PCM monitors the heater circuit for proper current draw and verifies that the downstream O2 sensor detects the expected temperature rise within a calibrated time window after a cold start. This monitor is only present on vehicles equipped with electrically heated catalysts, which are less common than standard catalysts.

Why It Matters for Emissions

Up to 80% of total trip emissions are produced in the first 60-90 seconds before a conventional catalyst reaches light-off temperature (~450-600°F). Electrically heated catalysts reduce this cold-start emissions window by 30-50%, making their heater function critical for meeting stringent SULEV and PZEV emissions standards.

Drive Cycle Steps

1

Begin with a cold engine — the vehicle must have soaked for at least 6 hours or until the engine coolant temperature is below 100°F (38°C), ideally near ambient temperature.

2

Start the engine and let it idle for 60-90 seconds; the PCM will command the catalyst heater on during this phase and monitor current draw.

3

Drive gently at moderate speeds (25-40 mph) for the next 3-5 minutes while the PCM monitors the rate of catalyst temperature rise via the downstream O2 sensor.

4

Avoid heavy throttle application during the first 5 minutes, as the PCM may abort the test if exhaust temperatures rise too quickly from combustion heat alone, masking the heater's contribution.

Prerequisites

  • Engine coolant temperature must be below 100°F (38°C) at startup — a true cold start is mandatory.
  • Battery voltage must be above 11.5 volts to ensure the catalyst heater element receives adequate electrical power.
  • No active DTCs related to the O2 sensor heater circuits, downstream O2 sensors, or catalyst heater relay.
  • Ambient temperature should be above 20°F (-7°C) for the monitor enabling criteria to be met on most vehicles.

Common Failure Reasons

  • Burned-out or high-resistance catalyst heater element due to thermal cycling fatigue over time.
  • Corroded connector or damaged wiring at the catalyst heater harness, often from road salt and moisture exposure.
  • Failed catalyst heater relay or blown heater circuit fuse preventing power delivery to the heating element.
  • Insufficient battery charge or failing alternator unable to supply the 10-20 amp heater current draw during cold start.

Pro Tips

  • This monitor is relatively uncommon — most gasoline vehicles use close-coupled catalysts near the exhaust manifold instead of electrically heated catalysts. Check the vehicle's emissions label to confirm if this monitor applies.
  • You can verify heater circuit operation with a clamp-on amp meter around the heater power wire during a cold start — expect 10-20 amps for the first 60-120 seconds.
  • If this monitor shows 'not supported' on the scan tool, the vehicle simply doesn't have a heated catalyst and the monitor is not required for emissions testing.
  • On hybrid vehicles, the heated catalyst is especially important because the engine may not run continuously, making combustion-based catalyst heating less reliable.

Frequently Asked Questions

What does the Heated Catalyst Monitor check?

The Heated Catalyst Monitor evaluates the performance of catalytic converters equipped with electric heating elements designed to bring the catalyst to light-off temperature more quickly during cold starts. The PCM monitors the heater circuit for proper current draw and verifies that the downstream O2 sensor detects the expected temperature rise within a calibrated time window after a cold start. This monitor is only present on vehicles equipped with electrically heated catalysts, which are less common than standard catalysts.

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

Follow the drive cycle: Begin with a cold engine — the vehicle must have soaked for at least 6 hours or until the engine coolant temperature is below 100°F (38°C), ideally near ambient temperature. Start the engine and let it idle for 60-90 seconds; the PCM will command the catalyst heater on during this phase and monitor current draw. Drive gently at moderate speeds (25-40 mph) for the next 3-5 minutes while the PCM monitors the rate of catalyst temperature rise via the downstream O2 sensor. Avoid heavy throttle application during the first 5 minutes, as the PCM may abort the test if exhaust temperatures rise too quickly from combustion heat alone, masking the heater's contribution. Estimated completion: Requires a cold start with 5-8 minutes of gentle driving. The cold soak requirement means this monitor can typically only be tested once per day.

Why does the Heated Catalyst Monitor keep failing?

Common failure reasons include: Burned-out or high-resistance catalyst heater element due to thermal cycling fatigue over time.; Corroded connector or damaged wiring at the catalyst heater harness, often from road salt and moisture exposure.; Failed catalyst heater relay or blown heater circuit fuse preventing power delivery to the heating element..

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Quick Info

Type

Non-Continuous

Completion

Requires a cold start with 5-8 minutes of gentle driving. The cold soak requirement means this monitor can typically only be tested once per day.

Resets on Clear

Yes