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Old 11-20-2017, 02:41 PM
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Originally Posted by gardentractorworld View Post
I'm new here to the forum and I have a question about my 98 YMCA sierra 2500 2wd with a 6.5 diesel. Every time I drive the truck I get a P0400 and P0501 code for Egr flow and Egr flow insufficient. The truck is all stock other than I cut the muffler and converter off and put flex pipe on instead. I have done a few tests and I have vacuum to the first two black solenoids on the drivers side while the truck is ideling but I'm not getting any vacuum coming out of the middle solenoid. I checked all the vacuum lines and none of them are collapsed and they all look good. Does anyone know why I keep getting these codes. They come on after 20 or 30 minutes of driving and is it possible the codes are on because I removed the converter and muffler? Any help would be greatly appreciated.
P0400 - EGR Flow Malfunction
P0401 - EGR Flow Insufficient
P0501 - Vehicle Speed Sensor (VSS) Circuit Malfunction

The only real component in the system that can monitor EGR flow is the MAF sensor. There is no differential EGR flow detector like those used with gas engines.

So, the question is, what can cause EGR flow DTC codes?

A bad MAF sensor.

A bad vacuum pump or loss in vacuum (leaking lines/hoses), which means the EGR valve won't open.

A bad EGR solenoid, which is a vacuum switch that controls the EGR valve.

A bad turbocharger wastegate solenoid, which can affect incoming airflow over the MAF sensor.

A plugged catalytic converter will restrict exhaust flow resulting in excessive backpressure in the exhaust system, which reduces the intake airflow read by the MAF sensor. Removing the cat/muffler could still produce EGR flow codes because the EGR system was calibrated to work as a complete system. A total loss of restriction would alter the flow dynamics.

Always begin a troubleshooting process by carefully examining the related wiring and electrical connectors. Some these tests can be performed using a Tech II scan tool from the driver's seat. Might be worth the 1/2-1 hour of shop time at a dealership if the tech knows these diesels.


1- Measure the vacuum.
2- If vacuum pump and lines are OK, replace EGR solenoid.
3- If that didn't help, replace wastegate solenoid.
4- If that didn't help, measure exhaust system pressure differential across the catalytic converter to determine whether the cat is plugged. Replace cat if above about 3-psi at WFO (a hard run). Test this by drilling a 1/8" hole in the exhaust system right before/after the cat, install pressure fittings, do pressure test, then weld holes closed if pressure is OK. Most cats slowly plug over time. A few years of driving is enough to completely plug a cat when the vehicle is lightly driven in cooler climates.

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Old 11-21-2017, 08:55 AM
Kennedy Kennedy is offline
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When the EGR valve opens you are recycling so the ECM expects a reduction in MAF readings. With the exhaust opened up there is slightly less back pressure, greater gross flow and less EGR flow.

This can all be taken care of in the ECM programming.
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Old 11-25-2017, 11:22 AM
gardentractorworld gardentractorworld is offline
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I haven't cleaned the egr valve yet it didn't look that bad. I was assuming it had something to do with taking off the cat and muffler.
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