6.2 Catalytic Converter Operation and Diagnosis
Key Takeaways
- A three-way catalyst simultaneously oxidizes HC and CO and reduces NOx, which requires fuel control cycling around stoichiometric 14.7:1.
- The substrate is a cordierite ceramic honeycomb coated with platinum and palladium (oxidation) plus rhodium (NOx reduction).
- Light-off temperature is roughly 500°F, and 60-80% of regulated tailpipe emissions occur during the first 90 seconds after a cold start.
- P0420 and P0430 are set when the downstream O2 sensor begins mimicking the upstream sensor, indicating loss of oxygen storage.
- Misfire, rich/lean fueling, oil consumption, and coolant ingestion destroy converters — the upstream cause must be fixed before replacement.
A modern gasoline vehicle cannot meet Tier 3 / LEV III tailpipe limits without a healthy three-way catalytic converter (TWC). The converter is the single most expensive emissions component on the vehicle, and the L1 exam expects you to know exactly how it works, how the PCM monitors it, and what destroys it.
What "Three-Way" Means
A three-way catalyst performs three simultaneous chemical reactions on the exhaust stream:
- Oxidation of HC → CO2 and H2O
- Oxidation of CO → CO2
- Reduction of NOx → N2 and O2
Oxidation needs a small amount of free oxygen; reduction needs a slightly oxygen-starved (rich) environment. The only way to do both at once is to keep the air-fuel ratio cycling rapidly around stoichiometric (14.7:1) so the catalyst sees alternating lean and rich pulses. This is exactly why the front O2 sensor switches several times per second in closed loop — the PCM is deliberately dithering fuel to keep the converter happy.
Internal Construction
| Layer | Material | Job |
|---|---|---|
| Substrate | Cordierite ceramic honeycomb (some performance units use metallic foil) | Provides massive surface area in a compact volume |
| Wash coat | Aluminum oxide (Al2O3) and cerium oxide | Bonds the precious metals to the substrate; cerium also stores and releases oxygen |
| Precious metals | Platinum (Pt), Palladium (Pd), Rhodium (Rh) | Pt and Pd handle oxidation of HC and CO; Rh handles NOx reduction |
A typical light-duty TWC contains only 3-7 grams of precious metal, but it is what makes the unit cost hundreds of dollars and what makes catalytic-converter theft profitable.
Light-Off and Why Cold Starts Dominate Emissions
A cold catalyst does almost no useful work. The converter must reach its light-off temperature — about 500°F (260°C) — before conversion efficiency climbs above 50%. Roughly 60-80% of all regulated tailpipe emissions on a properly maintained vehicle leave the pipe during the first 90 seconds after a cold start, before light-off. That fact drives most of the strategies you will see on the L1 exam:
- Close-coupled converters mounted within inches of the exhaust manifold
- Retarded ignition timing during warm-up to heat the exhaust
- Secondary air injection to provide O2 for HC oxidation while the cat warms
- Fast-acting heated front O2 sensors so closed loop starts quickly
The Catalyst Efficiency Monitor
OBD-II monitors the converter indirectly with two oxygen sensors. The L1 exam loves this comparison.
| Sensor | Position | Healthy Pattern in Closed Loop |
|---|---|---|
| Upstream (B1S1) | Before the catalyst | Switches rapidly between ~0.1 V and ~0.9 V, several times per second |
| Downstream (B1S2) | After the catalyst | Sits nearly flat, rich-biased at about 0.6-0.8 V, with very little activity |
The PCM compares the switching activity of both sensors. A healthy converter stores and releases oxygen via the cerium wash coat, so the downstream sensor sees a smoothed signal. A failed converter cannot store oxygen, so the downstream sensor begins to mimic the upstream sensor — that is the trigger for P0420 (Bank 1 catalyst efficiency below threshold) or P0430 (Bank 2).
What Kills Converters
A converter rarely dies of old age. The L1 exam will give you a scenario where P0420 has set, and the right answer is almost always to find the upstream cause:
- Misfire — raw fuel ignites inside the converter, melting the substrate. This is why most P0300-family codes inhibit the catalyst monitor.
- Rich mixture — sustained rich operation overheats the cat and sinters the precious metals.
- Lean mixture — chronic lean operation lets free oxygen overwhelm the reduction side and bakes the substrate.
- Oil consumption — phosphorus and zinc from engine oil poison the catalyst surface.
- Coolant ingestion — silicates from coolant glaze the wash coat.
- Leaded fuel or fuel additives — lead permanently disables precious-metal sites.
Replacing the cat without fixing the cause guarantees a repeat failure within months.
Backpressure and Restriction Testing
A clogged or partially melted converter creates exhaust restriction that mimics a no-power complaint. Two L1-approved tests:
- Vacuum gauge at idle: a healthy engine holds a steady 17-21 inHg. If you snap the throttle and the gauge drops then rises slowly back, restriction is indicated.
- Direct backpressure gauge screwed into the upstream O2 sensor bung or pre-cat port: should read less than about 1.25 psi at 2,500 RPM on most light-duty applications. Manufacturer specs vary; always confirm against service information.
If backpressure is high but the converter passes a tap test and shows no signs of melting, also inspect the muffler and any downstream resonator.
A scan tool shows the downstream O2 sensor (B1S2) switching nearly as fast as the upstream sensor (B1S1) on a warm engine in closed loop. Fuel trims are within +-3%. Which condition is MOST consistent with this data?
A vehicle has a confirmed P0420 code. Long-term fuel trim runs +18% at idle and +14% at cruise. A smoke test reveals a vacuum leak at the intake manifold gasket. Which repair sequence is correct?