What's causing a '97 plymouth voyager minivan to get P0134 & P0401 diagnostic code faults with dash 'checklight' for service?

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Check crankshaft sensor.

What is wrong is that you changed a part

You failed to follow any trouble charts because
you have none

The sensor was most likely okey & just doing it's
job of reporting a fault

You could have a mechanical problem such as a timing
belt issue or worn VVT Actuators on the end of the cames

You have to diagnose code related issues & then find out
why a sensor reported as it did

Even if you scoped the sensor you would most likely
assume it was bad,not so,have to keep thinking why the
data was bad

You need professional tools & look at several pieces of data
overlayed on a big sceen at one time

Repair shop or dealer for that issue

Hello
The engine and transmission in this cars drive train are fully electronically controlled by a computer called the PCM (Power Train Control Module). Whenever a problem like this occurs the computer stores a record of the problem (there are of course some exceptions to this, like the fuel pump for instance) in the form of a fault code in its memory, to read these fault codes you must have the systems memory scanned with a special tool. Once the fault code(s) are read you then must perform the appropriate diagnostic testing to find and resolve the problem(s) DO NOT REPLACE ANY PARTS UNTIL THE DIAGNOSIS & TESTING IS COMPLETED.

The e g r valve is the problem,replace it to solve the problem.

WELL I AM LOOKING AT AUTOMATIC SHUT RELAY BAD OR THE BAD COIL POWER TRANSISTOR.YOU WILL NEED A SCANNER TO TELL YOU IF YOUR CMP SENSOR OR CKP SENSOR IS WORKING.

It sounds like the connection to the cluster is not making good contact.

16518- P0134
O2 Sensor Circ.,Bank1-Sensor1 No Activity Detected

p0401 means you have insufficient egr flow, what you will have to do is have the egr ports cleaned out. this will involve removal of the egr valve so it would be best to take it to your local dealer where they will be able to fix ya right up.

There is a body control module that may cause this issue, it has the ability to be tested, have that done first.