P1421 – Cold start emission control – malfunction (Nissan)

By Reinier (Contact Me)
Last Updated 2023-04-03
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P1421 Mean?
2. Where is the P1421 sensor located?
3. What are the common causes of code P1421?
4. What are the symptoms of code P1421?
5. Get Help with P1421

What Does Code P1421 Mean?

OBD II fault code P1421 is a manufacturer-specific trouble code that is defined by carmaker Nissan as, P1421 – “Cold Start Control”, or more commonly on newer Nissan models, including all hybrid models, as P1421 – “Cold Start Emission Reduction Strategy Monitoring”, and is set when the PCM (Powertrain Control Module) detects a defect or malfunction in the system(s) that limit exhaust emissions when the engine is cold.

On OBD I- complaint Nissan vehicles, the sole functions of the cold start control system were to manage stable engine operation after cold starts by a) injecting an increased quantity of fuel into the cylinders to improve combustion, and b) managing the ignition timing to prevent premature ignition of the greatly enriched air/fuel mixture.

However, this situation changed after the introduction of the OBD II system in 1996. OBD II compliance required efficient and measurable reductions of exhaust emissions after cold starts, with the most important implementations of this requirement being the reduction of engine warm-up times, as well as the time it takes an emission control system, and by extension, the overall engine management system to enter into closed-loop operation. In this context, “closed-loop” operation occurs when the fuel and engine management systems use feedback signals from various engine and exhaust sensors to manage the operation of the engine, as opposed to using pre-programmed default values and settings stored in control modules to manage the engine’s operation.

In practice, though, depending on the engine, Nissan uses slightly different strategies to both manage and monitor exhaust emissions after cold engine starts, but these strategies largely use the same logic and basic operating principles. Here is a generic description of how this system works-

As soon as the engine starts, the PCM records the temperature of the engine coolant, with this value being the most important parameter the PCM uses to calculate an appropriate fuel delivery strategy to ensure stable combustion. At the same time, the PCM will adjust the ignition timing to prevent the premature ignition of the enriched air/fuel mixture, and supply power to the heating elements in the oxygen or air/fuel ratio sensors to reduce these sensors’ warm-up times.

If the vehicle is fitted with a modern, heated three-way catalytic converter, the PCM will also supply power to the converter’s heating element to reduce the time it takes for the catalysts to reach their optimal operating temperature. On modern vehicles, both the oxygen or air/fuel ratio sensors and the catalytic converter will reach their optimal operating temperatures within 10 to 20 seconds after the engine starts, which marks the point at which the emission control system enters closed-loop operation.

However, the proverbial devil always lives in the details, and in this case, the details involve the fact that it usually takes longer than 10 to 20 seconds for the engine to reach its optimal operating temperature. Therefore, since combustion is very inefficient at low engine temperatures, the PCM monitors the engine coolant temperature very closely, because combustion improves almost linearly with the rising engine temperature.

Put differently, this means that the engine management system will progressively reduce the amount of fuel it delivers to the cylinders as the engine gets progressively hotter. At the same time, the PCM will also advance the ignition timing closer to its base setting to improve combustion even further.

As a practical matter, the oxygen or air/fuel ratio sensors and the catalytic converter will usually be at their optimal operating temperatures when the engine coolant reaches a temperature of about 104 deg F, at which point combustion of the air/fuel mixture is typically stable and complete enough not to require enrichment of the mixture.

Thus, in a fully functional emission control system, the engine and fuel delivery systems will usually enter closed-loop operation when the engine coolant’s temperature reaches about 100 to 104 deg F, at which point the catalytic converter will begin to convert harmful components of exhaust gas into (mostly) carbon dioxide and water vapor. Note that the PCM records the elapsed time between a cold engine start and the moment the engine management system enters closed-loop operation for self-diagnostic purposes.

While the above is a grossly oversimplified description of how Nissan engines control exhaust emissions after cold starts, the point is that the PCM expects to see the moment the system enters closed-loop operation. However, since this must happen within a specified time, the PCM will recognize that there is a fault or defect present in the emission control system that prevents the system from entering closed-loop operation within the specified time. When this happens, the PCM will set fault code P1421, and illuminate a warning light, which may or may not flash, depending on the nature of the problem.

Where is the P1421 sensor located?

This image shows the placement (arrowed) of the catalytic converter in the exhaust system of a Nissan SUV. This is the heart of the emission control system on all Nissan vehicles, as such, the system that controls the emissions of cold engines is designed and programmed around the need to make the catalytic converter enter closed-loop operation as soon as possible after a cold engine start.

Note, though, that although modern emission control strategies are several orders of magnitude more advanced and effective than similar systems of just 10 or 15 years ago, it is still not possible to limit the exhaust emissions of (a modern) cold engine to the levels that are achievable when the engine is hot and in closed-loop operation.

What are the common causes of code P1421?

NOTE #1: TSB NTB09-116a addresses a programming problem that sets code P1421 on all 2007 – 2010 Versa models that are fitted with the MR18DE engine. The only remedy for code P1421 on these models (when there are no drivability issues or codes present)  is to have the PCM updated with the correct fix or “patch”, but be aware that this programming fix can only be performed by suitably qualified persons using Nissan-specific scan tools and software.

NOTE #2: TSB NTB10-089 addresses a similar programming issue on all 2009 – 2010 CUBE models. As above, the only remedy when no drivability issues or codes are present is to reprogram the PCM.

Apart from the above, some common causes of code P1421 could include one or more of the following-

• Dirty, damaged, contaminated, or defective throttle body
• Failure to perform a throttle body relearn procedure correctly after a throttle body service or replacement
• The use of a substandard aftermarket throttle body
• Defective or malfunctioning engine coolant sensor(s)
• Defective or malfunctioning oxygen or air/fuel ratio sensors
• A defective, or malfunctioning catalytic converter
• Damaged, burnt, shorted, corroded, or disconnected wiring and/or connectors almost anywhere in the emission and larger engine management systems
• Abnormally high or low fuel pressure
• Engine vacuum leaks, such as through defective or damaged intake manifold gaskets/seals
• Persistent misfires, but note that misfires can sometimes also be a symptom of code P1421, as opposed to being a cause of the code
• Insufficient intake airflow
• Defective or corrupted programming in the PCM

What are the symptoms of code P1421?

Common symptoms of code P1421 could include one or more of the following-

• Stored trouble code and an illuminated warning light
• Depending on the nature of the problem, multiple additional trouble codes could be present along with P1421; codes relating to catalytic converter efficiency and misfires are common additional codes
• Fuel consumption may increase noticeably
• The idling quality may be poor
• The engine may stall unexpectedly or repeatedly when running at idling speed, especially when the engine is cold
• Acceleration may be poor when the engine is cold
• The engine may be hard to start in very low ambient temperatures
• The engine may misfire severely when it is cold
• Catalytic converter failure could occur if this code is not resolved promptly
• The vehicle will fail an emissions test

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