Old School OBDII - EGR Flow Test - Aftermarket Parts
For anyone interested in seeing an early GM OBDII diagnostic routine in action, check out this video case study. It's not cutting edge, but it does illustrate how much data GM provided technicians way back in the early days of OBDII.
This tale was worthy of a case study in my book because what should have been a slam-dunk service, turned into a multi-visit comeback. Hopefully it might help someone in the future. Fortunately, this was a long time customer with a vehicle we've been taking care of since she bought it new.
The vehicle initially arrived with a P1404 and if you're still reading, watch the video for the rest of the story.
For reference, this is what the PCM runs for diagnostics on the P0401 according to SI:
CIRCUIT DESCRIPTION The PCM tests the EGR system during deceleration by momentarily commanding the EGR valve open while monitoring the Manifold Absolute Pressure (MAP) sensor signal. When the EGR valve is opened, the PCM should see a proportional increase in MAP. If the expected increase in MAP is not seen, the PCM notes the amount of error that was detected and adjusts an internal fail counter towards a fail threshold level. When the fail counter exceeds the fail threshold level, the PCM will set DTC P0401. The number of test samples required to accomplish this may vary according to the amount of detected flow error.
Normally, the PCM will only allow one EGR flow test sample to be taken during an ignition cycle. To aid in verifying a repair, the PCM allows a specified number of test samples during the first ignition cycle following a scan tool Clear Info or a battery disconnect. Between nine and twelve samples should be sufficient for the PCM to determine adequate EGR flow and pass the EGR test if the system is operating correctly.