From: Ray O on

"SMS" <scharf.steven(a)> wrote in message
> Ray O wrote:
>> "SMS" <scharf.steven(a)> wrote in message
>> news:4aa1893b$0$1586$742ec2ed(a)
>>> Just got the P0401 code on a 1996 Camry. I cleared it, but it came back
>>> the next day.
>>> I know what it says "Insufficient EGR Flow" but what is the procedure
>>> for finding the offending part? The vehicle has 130,000 miles and has
>>> never had an EGR valve replacement.
>> I've been posting on another thread for the same problem.
>> If you have a Mighty-Vac hand held vacuum pump, apply vacuum directly to
>> the EGR valve while the engine is idling. If the EGR is opening properly
>> and the valve and tube leading from the valve to the intake manifold is
>> not clogged, the engine should start to idle roughly and want to stall
>> when the EGR valve is opened. If the engine idles normally or almost
>> normally when you apply vacuum to the EGR valve, the valve is stuck,
>> clogged, and/or the metal tube between the EGR valve and intake manifold
>> is clogged. You can try removing the valve, and if it is caked with
>> carbon inside, you can try digging out the carbon inside the valve and
>> tube with a screwdriver or awl or just replace the valve.
>> If the engine does start to idle poorly or stalls when the EGR valve is
>> opened, then check the sensor on the output side of the EGR valve. The
>> sensor is either an EGR gas temp sensor which should sense the rise in
>> temperature that results when the exhaust gas flows from the EGR valve,
>> or the sensor is an EGR position sensor, which senses whether or not the
>> EGR valve is opening. Either way, you can put a volt meter on the sensor
>> terminals and you should sense a change in voltage when the valve opens.
>> If there is no change in voltage, sensor is bad. If there is a change in
>> voltage, then it is on to the next steps.
>> If the engine idles poorly when the EGR valve is opened, signifying that
>> the EGR valve is operating properly, and the sensor is good, check the
>> EGR modulator (google "Toyota EGR P Q R" to find instructions on checking
>> the EGR modulator or look for the thread where I posted instructions
>> recently. Don't worry if the instructions are for a different model or
>> engine - the check is the same.
> Thanks Ray. I did go out today and buy a Mity-Vac at Pep Boys so I can try
> this.
> I cleared the code yesterday and am waiting for it to re-occur. This is
> the second time I cleared it, the first time it re-occurred in about 10
> miles of driving, but now it hasn't happened again yet. Does the fact that
> it doesn't quickly cause the Check Engine light to come on again give any
> clues to whether it's the valve or the tube or the modulator?

You can also use the Mighty-Vac to bleed the air out of brake lines.

I haven't checked, but I am pretty sure that P0401 is a code with 2-trip
detection logic, which means that the computer has to see the same trouble
on 2 consecutive trips to illuminate or extinguish the malfunction indicator
light (MIL), also known as the "check engine light."

The first time the MIL came back on in 10 miles, I'd bet it was 2 short

To answer your question, the quickness or slowness of MIL illumination
doesn't give much of a clue, so I'd check in the order that I originally

As an aside, IMO, the advice from people say you should clear trouble codes
and wait to see if the MIL illuminates again because a problem could clear
itself up is not useful, and the people who advise clearing the codes don't
know as much about how OBD II systems work as they think they do. One of
the nice things about OBD II is that if a condition that causes the
electronic control unit (ECU, or engine computer) to illuminate the MIL
somehow clears up, the MIL will extinguish itself without someone having to
clear codes. For example, if a loose gas cap causes the MIL to illuminate,
tightening the gas cap will extinguish the MIL in 2 trips. (There is a
specific definition of a "trip," where basically every system and sensor
monitored by the OBD system has a chance to send a valid signal to the ECU.)

Clearing codes without checking what caused the codes does 2 things -

1) I delays diagnosis of the problem. The likelihood that an ECU gives a
false indication of a problem is so low that for practical purposes, it is
2) It can make more work for whoever is diagnosing the problem because the
baseline information stored in the ECU in conjunction with the vehicle's
operation history is lost.

Ray O
(correct punctuation to reply)