The Impact of Cold Starts on Emissions Test Results and How to Mitigate Them

Emissions testing stands as a critical pillar in the global effort to reduce air pollution from light-duty vehicles. Regulatory bodies like the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) implement standardized test cycles—such as the Federal Test Procedure (FTP) and the Worldwide Harmonized Light Vehicles Test Procedure (WLTP)—to measure pollutants including carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. One of the most challenging and influential variables within these tests is the cold start condition. A cold start occurs when an internal combustion engine is started after a prolonged soak period (typically six to twelve hours or overnight), during which the engine and its after-treatment systems have cooled to ambient temperature. During this initial phase, the engine, catalytic converter, and oxygen sensors are not yet at their optimal operating temperatures, significantly affecting emissions test results. This article explores the underlying mechanisms of cold-start emissions, their impact on regulatory testing, and practical strategies to mitigate their influence for both manufacturers and service technicians.

Understanding the Cold-Start Phenomenon

When a modern gasoline engine starts from a cold state, the engine control unit (ECU) enters an open-loop fueling strategy. In open loop, the ECU ignores feedback from the heated oxygen sensors (HO2S) because those sensors require a certain temperature (typically above 300°C) to produce a reliable voltage signal. Without closed-loop correction, the ECU enriches the air-fuel mixture—often targeting a lambda value below 1.0 (rich)—to ensure stable combustion despite cold cylinder walls, poor fuel vaporization, and increased friction from cold oil. This rich mixture produces excessive carbon monoxide (CO) and unburned hydrocarbons (HC). Additionally, the catalytic converter, which requires temperatures above 250–300°C for efficient conversion known as light-off, is largely inactive during the first 30 to 90 seconds of engine operation. As a result, a disproportionate share of the total emissions produced during the entire test cycle occurs within the first few minutes. Research from the EPA and SAE International indicates that up to 80% of total HC and CO emissions from a typical FTP cycle may originate from the cold-start phase.

How Cold Starts Influence Emissions Test Results

Emissions test cycles are designed to represent real-world driving, but they place a strong emphasis on the initial cold-start segment. For example, the FTP-75 cycle in the United States begins with a cold-start phase called Bag 1, which lasts 505 seconds. Following a 10-minute hot soak, a second 505-second bag (Bag 3) is run as a hot start. The results from Bag 1 are compared to Bag 3 to evaluate the cold-start penalty. During cold-start testing, a vehicle that may otherwise meet strict emission standards can produce readings significantly above certification limits.

Quantifying the Cold-Start Penalty

Laboratory studies consistently demonstrate that cold-start emissions can be several times higher than hot-start emissions for the same vehicle. For a typical port-fuel-injected (PFI) engine, CO emissions during the first 30 seconds of a cold start can exceed 10–20 grams per mile, while the stabilized hot-running phase may produce less than 0.5 grams per mile. Hydrocarbon emissions follow a similar pattern, with cold-start levels often reaching 1–3 grams per mile versus 0.05–0.1 grams per mile after warm-up. Even with modern exhaust after-treatment systems, the cold-start contribution remains a dominant factor in overall certification results. This is why CARB and the EPA have introduced Low Emission Vehicle (LEV) and Super Ultra-Low Emission Vehicle (SULEV) standards that impose stringent requirements on cold-start performance.

Factors Contributing to Cold-Start Emissions

Several interrelated factors determine the magnitude of cold-start emissions:

Engine Temperature at Startup

The bulk temperature of the engine block, cylinder head, coolant, and oil heavily influences fuel vaporization and combustion stability. At lower block temperatures (e.g., 0°C (32°F) compared to 20°C (68°F)), fuel droplets tend to condense on cold cylinder walls, increasing the need for over-fueling to ensure a combustible mixture. This leads to higher HC and CO output. Advanced engine designs with integrated exhaust manifolds and rapid warm-up coolant circuits can reduce this effect.

Ambient Temperature

Cold ambient temperatures exacerbate the cold-start penalty. At –20°C (–4°F) ambient, engine friction increases dramatically due to higher oil viscosity, and battery voltage drops, reducing starter motor speed and altering fuel injection timing. The fuel itself may be less volatile, requiring additional enrichment. Many regulatory certifications include a cold-temperature test (e.g., FTP at –7°C (20°F)) to ensure vehicles comply across climates. A study by the Coordinating Research Council (CRC) showed that HC emissions can be 2–4 times higher at –7°C compared to 24°C (75°F) during cold starts.

Fuel Mixture Adjustments

As noted, the ECU’s open-loop strategy relies on pre-programmed fuel maps that add extra fuel based on coolant temperature, intake air temperature, and engine speed. Older systems with simple cold-start injectors or carburetors used choke plates to richen the mixture; modern direct-injection systems can use multiple injections per cycle to better atomize fuel, yet still rely on enrichment. The duration and aggressiveness of the enrichment strategy are calibrated to balance drivability, emissions, and fuel economy, but they are a direct source of excess pollutants.

Catalytic Converter Temperature

The light-off temperature of a three-way catalyst (TWC) is typically between 250°C and 350°C. Until the catalyst reaches this threshold, it converts only a small fraction of CO, HC, and NOx. Even with electrically heated catalysts or close-coupled catalysts positioned near the exhaust manifold, the time to light-off is a critical window. In many vehicles, the catalyst does not achieve full conversion efficiency until 60–90 seconds after startup. This means that the pollutants produced during the initial minute are emitted almost untreated.

Test Cycle Impacts and Regulatory Implications

Emissions certification globally employs test cycles that start from a cold engine. The WLTP, which replaced the New European Driving Cycle (NEDC) in 2017, includes a cold-start requirement with a 23°C (73°F) ambient temperature and a standard soak time. Real-world driving emissions (RDE) regulations in Europe also require cold-start measurement using Portable Emissions Measurement Systems (PEMS). In the United States, the FTP-75 and the US06 cycle (which includes a hot start) test cold-start performance, while the SULEV and Partial Zero Emission Vehicle (PZEV) standards demand that vehicles emit very low levels of hydrocarbons (0.010 grams per mile NMOG) even during the first few minutes of operation.

Why Cold Starts Matter for In-Use Compliance

In-use emission testing programs, such as the EPA’s In-Use Verification Program (IUVP) and CARB’s Smog Check in California, often measure cold-start emissions to ensure that vehicles continue to meet standards throughout their warranty period. A vehicle that performs well on a dynamometer hot run may fail a cold-start test due to a degraded oxygen sensor or a weak battery that alters the cold-start fueling. Therefore, understanding cold-start behavior is essential for maintaining compliance and avoiding recalls.

Strategies to Mitigate Cold-Start Effects on Emissions Test Results

A range of engineering and procedural measures can reduce the influence of cold starts on test results, helping manufacturers achieve lower certification values and helping service technicians ensure vehicles pass in-use tests.

Preconditioning: Engine Block Heaters and Battery Warmers

Engine block heaters, which are common in cold-climate regions, circulate heated coolant through the engine prior to startup, raising the block temperature to near operating levels even in sub-zero conditions. This reduces the enrichment requirement and shortens the time to catalyst light-off. For emissions testing, some facilities use electric heaters to pre-warm the engine to a controlled temperature (e.g., 30–40°C (86–104°F)) before the test, eliminating the cold-start penalty. However, this practice is not allowed during official certification tests where the vehicle must soak at the prescribed ambient temperature to represent real-world conditions. Aftermarket block heaters and battery warmers, however, are ethical modifications for in-use improvement and for test preparation when allowed by local regulations.

Extended Warm-Up Periods: Idling and Low-Load Operation

Allowing the vehicle to idle for 30–90 seconds before beginning the emissions test can significantly lower the peak cold-start emissions. Idling at low load warms the coolant and exhaust system without the high fuel enrichment associated with acceleration. Many modern vehicles employ an idle speed step at startup: the engine rpm remains elevated (e.g., 1200–1500 rpm) for a short time to aid warm-up. Shops conducting emissions testing can advise customers to warm up their vehicle for a few minutes before arriving at the test station, but this depends on the test protocol. Note that the official certification tests require the start to occur after the soak, so idling before the test is not permitted in those contexts.

Advanced Engine Design and Fast Light-Off Catalysts

Automakers have invested heavily in technologies that directly target the cold-start window:

  • Close-Coupled Catalysts: Placing the three-way catalyst within inches of the exhaust manifold reduces the mass of metal that must be heated, enabling light-off in as little as 10–20 seconds. This is standard on many modern cars.
  • Electrically Heated Catalysts (EHC): An electric heating element embedded in the catalyst substrate can raise its temperature before the engine even starts. EHCs are used on some hybrid and high-end vehicles to achieve SULEV and PZEV standards.
  • Hydrocarbon Traps: Some vehicles incorporate a zeolite-based HC trap in the exhaust path that adsorbs hydrocarbons during the cold phase and releases them later when the catalyst is hot and can convert them. This is effective for reducing cold-start HC spikes.
  • Direct Injection with Multiple Injection Events: Gasoline direct injection (GDI) systems can inject fuel in multiple pulses during the compression stroke, improving mixture preparation and reducing wall wetting. This lowers the need for global enrichment, cutting cold-start HC and CO.
  • Exhaust Gas Recirculation (EGR) During Cold Start: While EGR is typically used to reduce NOx at high load, some engines introduced by Toyota and others use a small amount of EGR during cold start to stabilize combustion and reduce enrichment.

Optimized Calibration Strategies

Modern ECU software can implement sophisticated cold-start algorithms. For example, adaptive learn routines can adjust fuel trim based on the last several cold starts, compensating for fuel composition changes or component wear. Deceleration fuel cut-off during the warm-up phase can also reduce emissions. Additionally, some calibrations include a fast idle cam function that gradually reduces idle speed as coolant temperature rises, minimizing unnecessary fuel consumption and emissions. Manufacturers use advanced 1-D and 3-D simulation tools to model cold-start behavior and optimize the trade-off between drivability and emissions.

Modified Testing Protocols and Data Analysis

From the regulator’s perspective, adjustments to test procedures can also mitigate the cold-start effect without requiring hardware changes. The FTP-75, for instance, includes a 10-minute hot soak between Bag 1 and Bag 3, which allows some cooling of the catalyst. Some alternative methods normalize emissions over the entire cycle, reducing the weight of the initial phase. For in-service surveillance, regulators increasingly use bag mini-dilution and modal analysis to identify cold-start contributions and determine whether they are within acceptable bounds. Laboratories can also use a cold-start weighting factor to compare results between vehicles, provided it is carefully justified.

Vehicle Inspection and Maintenance

For vehicle owners and repair shops, ensuring that the engine is in good condition reduces cold-start emissions. Key items include:

  • Clean and correctly gapped spark plugs – misfires during cold start greatly increase HC.
  • Proper oil viscosity – using thinner oil (e.g., 0W-20) reduces frictional drag and helps the engine reach operating temperature faster.
  • Functional engine coolant temperature sensor – a faulty sensor can cause overly rich mixtures.
  • Battery health – a weak battery may not spin the engine fast enough, leading to poor atomization and excessive enrichment.
  • Oxygen sensor function – aged sensors can cause the ECU to stay in open-loop longer.

Regular maintenance, as outlined in the owner’s manual and recommended service intervals, directly mitigates the cold-start penalty and improves passing test results.

Real-World Case Studies and Data

A 2018 study by the Chinese EPA (Ministry of Ecology and Environment) tested 100 light-duty gasoline vehicles at 23°C and –7°C. The results showed that average CO emissions at –7°C were 3.2 times higher than at 23°C during the cold-start phase. A separate SAE paper (2014-01-1598) examined a fleet of PZEV vehicles and found that electric heated catalysts reduced cold-start NMOG emissions from 0.025 g/mi to 0.008 g/mi, well below the SULEV30 limit. In Europe, RDE testing with PEMS has shown that a cold start can contribute up to 40% of total NOx emissions in a typical urban trip of 10 km. These figures underscore the importance of addressing cold starts.

As the automotive industry transitions toward electrification, the cold-start problem evolves. Mild hybrids and full hybrids can use the electric motor to assist engine start, reducing the need for enrichment. Plug-in hybrid electric vehicles (PHEVs) can operate in electric-only mode for the first few miles, completely avoiding the cold-start emissions from the engine. Battery electric vehicles (BEVs) obviously have no combustion emissions, but they still face thermal management challenges. Nonetheless, for the foreseeable future, millions of conventional and hybrid vehicles on the road will continue to experience cold starts. Advanced after-treatment technologies, such as passive NOx adsorbers and electrically heated substrates, will become standard to meet increasingly stringent regulations like Euro 7 and EPA Tier 3.

Conclusion

Cold starts impose a heavy burden on emissions test results by temporarily elevating pollutant output far above stabilized levels. Understanding the underlying causes—engine temperature, catalyst light-off delay, and rich fuel mixtures—is essential for engineers, regulators, and technicians alike. Mitigation strategies range from advanced catalytic heating and engine calibrations to simple measures like block heaters and proper maintenance. As regulatory standards tighten, the industry continues to innovate, driving down the cold-start penalty. Ultimately, accurate emissions testing requires accounting for the cold-start phase, and effective mitigation ensures that vehicles represent their real-world environmental performance—not just their behavior after a long warm-up. By combining careful design with intelligent test protocols, we can achieve cleaner air from the very first turn of the key.

For further reading, consult the EPA’s vehicle emissions testing resources, CARB’s Advanced Clean Cars protocol, and SAE technical paper “Cold Start Emissions Reduction with Electrically Heated Catalysts” for deeper technical insights.