← Back to list

P0401 and P0403 Fault on 1999 Plymouth Voyager - EGR Circuit Failure Diagnosis

Model: 99 Plymouth Voyager Fault Code: P0401 Posted: 2007-01-05 18:41

I'm facing a persistent issue with my 1999 Plymouth Voyager (150,000 miles) that has triggered the Service Engine Light and displayed both P0401 (Insufficient EGR Flow) and P0403 (EGR Circuit Failure) diagnostic codes. Initially, I replaced the EGR valve — confirmed to be functional as vacuum application causes stalling and manifold vacuum is adequate. However, after replacement, the P0401 code persisted, and a new P0403 code appeared. I now suspect a fault in the EGR wiring harness or circuit. The system appears to rely on a vacuum modulator for feedback, which signals the Powertrain Control Module (PCM) when the valve opens. Since there is no dedicated EGR flow sensor on this Chrysler model, I'm wondering if the PCM uses the vacuum modulator as a feedback mechanism. I've reviewed my Haynes manual and found that the EGR circuit runs from the Power Distribution Center (PDC), with ground signal supplied by the PCM. I plan to perform continuity testing on the harness and verify resistance in the EGR solenoid — typical values are 40–80 ohms according to reference codebooks. Any suggestions for troubleshooting steps or known failure points would be greatly appreciated.

Related fault codes
P0401P0403
Comments (6)
Anonymous 2007-01-08 08:00

Did the replacement EGR valve include a vacuum modulator with an electrical connector? This component is critical for feedback to the PCM.

Anonymous 2007-01-08 08:31

Use a multimeter to test continuity in the EGR wire harness and check the resistance of the EGR solenoid. A healthy solenoid should read between 40 and 80 ohms. Verify this value using your vehicle's service manual or reference guide.

Anonymous 2007-01-08 09:44

Thanks for the input — yes, the replacement valve came with a vacuum modulator and electrical connector. I've cross-referenced the wiring diagram in my Haynes manual and confirmed that the EGR circuit is straightforward: power from the PDC, ground signal from the PCM. I'll proceed with continuity testing. I’ve seen some sources suggest differential pressure sensors as potential failure points (common on Ford models), but I don’t see any such sensor on this Chrysler platform. Is there a feedback mechanism in place? If not, how does the PCM determine EGR operation? Marc

Anonymous 2007-01-08 12:39

The vacuum modulator serves as the feedback device — the PCM detects when it closes and monitors changes in oxygen sensor readings to infer EGR flow.

Anonymous 2007-01-08 19:11

That makes sense. I originally assumed there would be a dedicated sensor, but now understand that the vacuum modulator provides the necessary feedback via pressure changes detected by the O2 sensor. I’ll test the harness continuity and attempt to clear the codes to see if the issue resolves. Thanks for the clarification! Discover more forum Porsche car Thanks everyone — I'll update with results.

Anonymous 2007-01-23 18:33

Just to share the resolution: it turned out that the original EGR valve was faulty, and the issue wasn't in the wiring or solenoid. After replacing the valve, I expected the codes to clear after a few drive cycles, but they didn’t. So I used a handheld OBD2 scanner to manually clear the codes. Since then, I’ve completed several drives and passed my CT emissions test (OBD check) — everything is now within specifications. Thanks for all the helpful advice!