99 Plymouth Voyager Check Engine Codes P0403, P0455, P0401, P0700, and P1782: Diagnosis & Fixes for EGR and Transmission Issues

Is the transmission shifting smoothly? Does the speedometer function properly? While EGR and EVAP codes are relatively straightforward, transmission-related issues require a thorough check before assuming any component failure.

@Comp: As an additional note, if the EGR sensor lacks reference voltage, it might be shorted or shared with another circuit — could this explain why multiple codes appear together? Just a thought.

It's possible that wiring issues are affecting both the EGR and LDP (Lift and Drive Pump) systems. The EGR valve and LDP are located close to each other in this model, and I've seen cases where shared wiring causes overlapping fault codes. However, the presence of a transmission code suggests the issue may have been present for some time — possibly pending until shifting problems emerged. More details from the owner would help rule out any major issues.

-k- Thanks for the input!

I removed and cleaned the EGR valve, also cleaned the EGR solenoid and the attached EET transducer. I replaced the PCV valve and hose (which was broken). After disconnecting the battery for a few minutes, I drove the car — but the check engine light returned. Today, after extensive driving, I scanned again and only saw P0403. The scanner indicated either the EGR vacuum solenoid or EGR pressure sensor is faulty. No other codes were present.

The EGR solenoid is part of the new valve assembly — without a proper scan tool, you can't verify if the PCM is sending signals to the transducer. EGR systems are common failure points! The P0455 code may return because it's a two-trip or multi-trip monitor that depends on fuel tank level and ambient temperature. No repair will permanently fix this unless the leak source is addressed. I strongly recommend having the transmission inspected by a professional shop.

I'm still searching for the large evaporative leak. The smell of gasoline appears when I stop driving — it seems to come from the rear of the vehicle. I checked the hose from the fuel cap to the tank — it's slightly rusted but no visible holes. I also inspected all undercarriage hoses and found nothing suspicious. I'll scan again soon to see if codes return, and I’ll have the transmission checked as well.

Please keep us updated on your progress!

A visual inspection alone isn't sufficient to detect EVAP leaks. There are hoses located above the fuel tank and in hard-to-reach areas that may be compromised. Each hose should be individually inspected. Also, as Comp mentioned repeatedly, LDP pumps can leak — this is a common cause of gross evaporative system leaks.

I had the EGR replaced and discovered that the 'gross leak' was caused by a defective fuel cap. I used an original OEM cap from the dealer — aftermarket caps didn’t resolve it. The gas smell disappeared after this fix. However, the check engine light returned after 150 miles of mixed driving. After re-scanning, I now see P1784 (Transmission-related sensor fault). While it's labeled as a sensor issue, I'd like to know which one exactly and whether it can be fixed at home. Thanks for your help.

Aftermarket fuel caps may have different valve designs — could this affect performance? Also, did the shop provide a code description? That would greatly assist in diagnosing the issue!

No, they didn’t give any explanation. If it's a speed sensor fault, is it easy to replace?

Speed sensors are relatively simple to replace — but only after confirming the exact code and having a clear diagnosis from the scan tool. If no description is provided, I wouldn’t pay for the work! Does the transmission shift normally? Does the speedometer function?

The transmission shifts fine. I removed both input and output speed sensors and noticed metal dust on them — I cleaned them thoroughly. After reinstalling, I also changed the transmission fluid and filter. The check engine light stayed off for about 250 miles before returning. Now I see P0401 and P0700 again. Possible causes: P0401: 1) Blocked EGR system; 2) Failed EGR valve; 3) Faulty EGR solenoid; 4) Failed EGR pressure sensor (if applicable); 5) Failed EGR position sensor (if applicable). P0700: Automatic transmission fault. The ECM detected a failure in the transmission control module. Possible causes: 1) Short or open circuit within the transmission; 2) Failure of the transmission control module. Additional note: The system may trigger an 'AT MIL light up request' — this could indicate a service code stored in the TCM.

A more advanced scan tool would be able to access deeper transmission fault codes and verify EGR solenoid commands. If you apply vacuum to the new EGR valve, does the engine stall? Were both the EGR valve and modulator replaced?

After digging into P0700 with a better scanner, I found P1782 (Pressure Switch Circuit Out of Self-Test Range) — likely meaning 'pressure switch circuit' malfunction. My friend had this tool but wasn’t certain about the exact meaning. We also received code 18 ('Service Manual'), and for P0401 we saw 'EGR System Performance.' Is it 'system performance' or 'perfusion'? Since the EGR was replaced, could a blockage in one of the hoses or pipes be causing this? Could it be a perforation issue?

For the EGR system, I suspect a wiring fault with the solenoid. For the transmission code P1782, I recommend taking it to a professional transmission shop for diagnosis and repair.