Posted by Alex (Im) E. on 01 February 2013 01:34 AM
On some vehicles, the OBD II system may monitor the operation of the exhaust gas recirculation system.
The EGR system reduces the formation of oxides of nitrogen (NOX) in the exhaust when the engine is under load.
OBD II may check for a drop in intake vacuum through the MAP sensor when it commands the EGR valve to open.
Some systems also have an EGR valve positon sensor to monitor the opening and closing of the EGR valve.
Code for EGR-related faults include P0078 to P0086, and P0400 to P0409.
An EGR-related fault code does NOT mean the vehicle is polluting. It might be producing elevated levels of NOX in the exhaust if the EGR system is not operating correctly.
The only way to know for sure is to check NOX emissions with an exhaust analyzer.
NOX emissions are highest when the engine is under load, which means you may not be able to get accurate readings unless the vehicle is tested in a "loaded mode" by driving it on a dyno, or using a portable exhaust analyzer while test driving the vehicle on the road.
MORE ABOUT THE EGR SYSTEM
Exhaust gas recirculation is only a part-time function. It should NOT occur when the engine is cold because it acts like a vacuum leak and can cause a rough idle or lean misfire.
EGR also should also NOT occur at idle for the same reason. It should only occur after the engine has warmed up and is running at a speed above idle.
The EGR valve should be fully closed when the engine is off. If it is not, it can make for hard starting when the engine is cranked.
On most OBD II vehicles, the EGR valve is controlled by the powertrain control module. If teh engine has a vacuum-operated EGR valve, the PCM controls a solenoid in the vacuum line to open and close the valve.
The PCM may cycle the solenoid on and off to vary the EGR flow rate.
Inceasing the "on time" of the solenoid holds the valve open longer and increases the flow rate.
"Digital" EGR valves have several solenoids (two or three in the case of GM). Each has a different sized valve (small, medium and large).
The PCM varies the flow rate by energizing various combinations of the solenoids. The solenoids are normally closed, and open only when the PCM completes the ground to each.
If an engine has an electronic EGR valve, the PCM operates the valve by energizing a motor or solenoids.
"Linear" EGR valves use a motor to open the valve. The further the valve opens, the greater the flow rate. Linear EGR valves may also be equipped with an EGR valve position sensor (EVP) to keep the PCM informed about what the EGR valve is doing.
The EVP sensor also helps with self-diagnostics because the computer looks for an indication of movement from the sensor when the it commands the EGR valve to open or close.
The sensor works like a throttle position sensor and changes resistance. The voltage signal typically varies from 0.3 (closed) to 5 volts (open).
GM FALSE EGR CODE P1406
On some 1995 and newer GM vehicles with OBD II and linear EGR valves, the MIL light may come on and set a Code P1406 when the engine has high mileage on it.
The P1406 code indicates an EGR problem, but replacing the EGR valve won't fix it because the real problem is that the OBD II system's self-diagnostics are overly sensitive.
When the computer commands the EGR valve to open, it wants to see a movement signal from the EVP sensor within a certain number of milliseconds (typically 50 ms or less).
But as the EGR valve ages, it may not open as quickly as it once did. It still works and keeps NOX emissions within acceptable limits, but the computer thinks the EGR valve is not opening quickly enough and sets a code.
The cure, in this case, is not to replace the EGR valve but to reflash the PCM with new programming instructions that allow a longer response time from the EGR valve. This requires a Tech 2 scan tool and a software update from GM.
MODE 06 SCAN TOOL DIAGNOSTICS
Most scan tools can access a menu called "Mode 06." You will usually find this by choosing "global" or "generic" OBD II on the scan tool main menu rather than entering the vehicle year, make and model.
When you go to the Mode 06 menu, you can see the OBD II self-test data for all the EGR components and can tell at a glance whether they are operating in or out of range. If a component is acting up, it will FAIL the self-test -- but it may not set a DTC unless all the other failure criteria for the DTC have also been met.
This may take several drive cycles to occur.
See the Mode 06 Diagnostics section for more info.
COMMON EGR PROBLEMS
Common EGR problems include:
GENERAL EGR TROUBLESHOOTING TIPS
There are several ways to troubleshoot an EGR system. If the MIL lamp is on and there is anOBD II code that is related to the EGR system, you can refer to the diagnostic charts in the vehicle service manual to isolate teh fault. If you don't have access to the service information, you can use the following procedure:
TIP: Tune Your Car with Powerful Diagnostics Software
Whether you're a home car owner or an auto mechanic — you can save thousands of dollars on car maintenance, save time and effort on repairs, and dramatically improve your cars lifespan/performance — by getting yourself a professional car diagnostics scanner.
They're called OBD scan tools. And will give you an instant overview of your vehicles condition by tapping into it's computer chip.
But more importantly, you can also obtain software that will allow you to edit the cars performance data. Like adjusting RPM, fuel injection, etc. This is what car performance shops charge thousands of dollar for — to allow the car to operate more fuel efficiently, faster or with stronger torque/BHP.
The good news is, you can do the same at fraction of a cost, with a USB connection interface to your laptop, and car tuning software. We suggest you Google on this to discover more. Or you can get this car diagnostics/tuning package here.