Generic OBDII P Codes P0400-P0499

Learn the code definitions and most common causes for Generic OBDII P Codes P0400-P0499

This post lists the most common trouble codes in the OBDII P Codes P0400-P0499 category. These are Auxiliary Emissions-related trouble codes. Start by reading the most common codes and causes, and then refer to the code chart at the end of the post for official code definitions.

How to find your P Codes P0400-P0499 definition

There are 170 400-series codes. There are several ways to find the code definition in the chart below.
1) Enter the trouble code in the search box. That’s the fastest way
2) If you’d like to scroll all the codes, change the Number of entries to show on the page in “Show Entries” box. Click NEXT or PREVIOUS at the bottom right of the table

This graphic shows the three ways you can navigate the refrigerant chart: 1) Increase the number of entries, enter the year of your vehicle in the search box, or scroll the chart using the next/previous buttons at the bottom of the chart

The most common causes of OBDII P Codes P0400-P0499

P0400 – P0409 Exhaust Gas Recirculation (EGR) trouble codes are very common

Carmakers use exhaust gas recirculation to cycle some of the exhaust gasses back into the combustion chamber to reduce combustion temperatures and reduce oxides of nitrogen emissions. The ECM commands the EGR valve to open when certain conditions are met. Depending on the vehicle, the valve can open in small increments Based on engine vacuum, a varying voltage, or a digital signal.

When the ECM commands EGR, it expects to see a change in engine rpm and a return signal from the EGR valve confirming that the valve opened as much as the ECM commanded. The EGR valve reports this based on an internal position sensor. The sensor is a common source of failures. In many cases, especially in Chrysler vehicles, the positioned sensor fails to report the correct amount of opening in the EGR valve.

Diagnosing an EGR-related trouble code

The most common cause of an EGR related trouble code

This is an electrically operated EGR valve with a position sensor mounted on the top of the valve. The sensor is a high failure point

is a failure of the valve itself or the position sensor inside the valve. In either case the valve must be replaced. Lots of DIYers have tried to clean the valve, but have very little success because the problem usually isn’t caused by carbon buildup.

Start by checking for corrosion in the electrical connector to the EGR valve. If that looks good, remove the EGR valve and check for a clogged exhaust port directly under the EGR valve. If the port is clear, replace the EGR valve.

For more information on how an EGR valve works, see this article
For more information on a P0400 code see this article
For more information on a P0401 code see this article
For more information on a P0404 code see this article
For more information on how the EGR readiness monitor works see this article

P0410 – P0419 Secondary Air codes are common

Carmakers often use secondary air during cold startups

to help the catalytic converter get up to full operating temperature faster. During a cold start, the engine needs a rich mixture. However, a good portion of that extra fuel winds up in the catalytic converter. That causes a situation where the catalytic converter is filled with fuel but doesn’t have enough oxygen or heat to burn it off. To correct the situation and reduce cold start emissions, carmakers add an air injection reaction (A.I.R) pump and a set of valves to pump outside air into the exhaust manifold, thereby introducing extra oxygen for the catalytic converter.

The most common causes of secondary air trouble codes

The secondary air system consists of an electrically driven air pump, solenoid switching valves and tubing. The switching valves are used to allow air to enter the exhaust manifold when the system is in use but close off when secondary air is no longer needed. When the valves are closed exhaust can’t back flow through the air pump.

The air pump and the valves are common failure points. Moisture from humid weather can cause corrosion inside the valves and the pump.

Diagnose secondary air components

The easiest way to diagnose secondary air trouble codes is to use jumpers to provide power and ground to the air pump and the switching valves to see if they operate properly. If not they must be replaced.

These are the three common components of a secondary error system

For more information on how secondary air systems work see this article

For more information on the secondary air readiness monitor see this article

P0420 & P0430 Catalytic Converter Codes are very common

These two codes indicate that the ECM has tested the catalytic converter and determined it is not operating properly based on information from the downstream oxygen sensor. The most common mistake DIYers make when diagnosing this code is to assume that the cause is a bad upstream oxygen sensor. Ninety-nine percent of the time, these codes are not caused by a faulty oxygen sensor. On the contrary, the ECM needs to see good data from the oxygen sensors in order to test the catalytic converter. If the upstream oxygen sensor were bad, the ECM would set an oxygen sensor code, not a catalytic converter code.

How to diagnose a P0420 or P0430 catalytic converter code

High fuel trims can cause these codes

If the ECM is adding too much fuel to the engine, that excess fuel will wind up in the catalytic converter and cause a P0420 or a P0430 trouble code. So, your first step is to look at the long-term fuel trims in the live data of a scant tool. If fuel trims are above 10%, find the cause and fix it before proceeding to diagnose the catalytic converters any further.

An exhaust leak can cause these codes

If the long-term fuel trims are in the normal range, your next step is to check for an exhaust leak. Any unmetered air entering the exhaust ahead of the catalytic converter can cause a P0420 or a P0430 code.

Older Subarus are the exception

The ECM’s in older Subaru vehicles can set a P0420 code due to a faulty oxygen sensor. This is because the software in older Subarus is not set up to test the upstream oxygen sensor. Subaru is the exception to the rule that a catalytic converter code would never be caused by a bad oxygen sensor.

For more information on how the ECM sets a P0420 or a P0430 code, see this article
For additional help in diagnosing a catalytic converter code, see this article

P0440 – P0449 Evaporative Emissions Codes are very common

The evaporative emission system reduces air pollution by absorbing fuel vapors that would otherwise escape into the atmosphere when filling your tank with gas. The system includes a charcoal canister, a purge valve, and an event valve. During fillup, the fuel vapors are absorbed by the activated charcoal inside the charcoal canister. After the fillup, the ECM opens the purge valve and the vent valve. The engine vacuum then sucks the fuel vapors out of the charcoal canister, replacing it with fresh air that enters through the vent valve.

Charcoal vapor canister

The most common causes of Evaporative Emissions Codes

• A stuck open or stuck closed purge valve is a common cause of evaporative emissions codes.
• A stuck open or closed vent valve is also a common cause of evaporative emissions codes.
• A damaged hose from the fuel tank to the canister or a damaged hose from the canister to the purge valve can cause a leak which sets an evaporative emissions code.
• A damaged charcoal canister A damaged charcoal canister can cause emissions codes but this is not a common failure point.

Diagnose an evaporative emissions code

P0441, P0443, P0444, and P0445 are purge valve codes. The Problem can be with the valve itself or the wiring to the valve. Using jumper wires for power and ground, operate the purge valve to see if it opens and closes. Next, use a hand-held vacuum pump to apply vacuum to this purge valve to see if it holds. For most of these codes the problem is a faulty purge valve.

P0445, P0446, P0447, P0448, P0449 are vent valve codes. The vent valve operates much like the purge valve but it is prone to clogging do to Spider nests in thee valve inlet. Start your diagnosis by checking for spider nests in the vent valve inlet. If the inlet is clean, use jumper wires for power and ground to operate the vent valve. If the valve fails to open or close replace it.

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