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Aaron, Nissan Technician
Category: Nissan
Satisfied Customers: 1
Experience:  ASE Master Tech, Certified Nissan Tech, High Performance Enthusiast
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1996 Nissan Maxima: check engine light..EVAP..control valve..the ECM

Customer Question

The check engine light on my 1996 Nissan Maxima stays on despite replacing EVAP Cannister & vent control valve, purge cut valve, purge monitoring switching valve, and purge control valve. I was told by Nissan that the ECM has failed does not ground causing the valves not to function properly. Please help me understand.
Submitted: 8 years ago.
Category: Nissan
Expert:  Aaron replied 8 years ago.

Hi there!


I was wondering what codes are stored for the engine light. I am assuming these are all evap system codes from your description. And, were all these repairs done at the same time? A little more info would help me understand the situation a little better.



Customer: replied 8 years ago.
09/18/08, 09/23/08, and 12/12/08 code P0443 Purge Control Valve. The purge control valve assembly was replaced on 09/18/08. On 09/09/08 code P1447 EVAP purge flow monitor and the purge monitoring switching valve was replaced that same day. A service advisor had me bring my car back yesterday (12/12/08) to confirm the diagnosis of a bad ECM. Found code P0443 purge control valve, found ECM not grounding cannister purge control solenoid curcuit, possible charcoal contamination in purge volume control valve (claimed that this was a different valve so far not replaced), boost sensor should be replaced due to charcoal contamination throught the whole system.

All the repairs that have been done were done on separate visits from 07/08 through 09/08.
Expert:  Aaron replied 8 years ago.

So Let me organize this so I have it strait. It will help me go over their diagnosis to understand the situation better and to help me give you a better understanding of what they did or did not do.


Starting -


You had a P1447 code on 9/9/08, which was fixed by replacing Purge Monitoring Switching Valve you said.


However, I do not show a Purge Monitoring Switching Valve on this vehicle.

Please explain what was replaced or repaired. The name of this part does not show in the Nissan Service Manual.


Secondly -

You had a P0443 Code following that on 9/18/08, which is when they replaced the Purge Control Vale assembly.


This code then came back again on 9/23 and again on 12/12 when you brought it in last.


Now they say that you need to replace the ECM because it is not grounding the Canister Purge Control Valve, and that you have possible contamination of the system with charcoal and may need to replace the Purge Volume Control Valve and the "boost sensor".


If this is correct, please let me know. If you have more information, please fill in. I will try to elaborate on what may be going on in their diagnosis and give you my opinion on the issue. Also, if you want I can elaborate on or explain the possibilities that each code presents so that you can understand the information at hand. This is a complex situation but I will try my best to explain possibly what they did or should have/ should do. Thanks

Customer: replied 8 years ago.
On 09/9/08 part #14956-38U01 Valve Assy -Sole Price $119.74.

That is correct regarding the ECM based on what they told me and was documented on the invoice.

I do have more information to give you. On 09/23/08 I took my car to a second Nissan service shop for a second opinion. They ran consult and code P0443 was found performed test on EVAP system found EVAP pressure sensor not responding. They had me prepay for a valve assy 14956-35U10 and had me return 09/26/08 to install it which they did. On 09/26/08 invoice, the valve assy 14933-54U0A showed a different part number from the invoice on 09/23/08. I left their shop drove my car a few miles and the check engine light came back on. I drove back to their shop was told to come back 10/3/08 which I did and was told the ECM needed to be replaced. They refunded me for the valve assy 14956-35U10 because they agreed that part was not needed.
Expert:  Aaron replied 8 years ago.

Ok thank you for adding the information. This is what I have concluded then with what I believe is going on.


Both Nissan shops have concluded that the ECM is not grounding the Canister Purge Control Solenoid Valve from what I understand. And both are telling you that the ECM must be replaced. Here is what I believe happened.


Firstly you had a P1447 code on 9/908. Which was fixed by replacing I believe (I can not cross check the Nissan part numbers on the weekend) the Canister Purge Control Valve. I am not sure if that part number is XXXXX this specific valve but either way a valve was replaced. Let me explain this code to you to help you understand.


P1447 - EVAP control system purge monitoring

This malfunction is detected when one of two things happens. The EVAP control system itself does not operate properly, or when the system has a leak in the line between the intake manifold and the EVAP Control System Pressure Sensor.


-Possible Causes for P1447 are:

Canister Purge Volume Control Valve stuck closed

Canister Purge Control Valve stuck closed

EVAP control system Pressure Sensor

Loose or disconnected rubber tube

Blocked rubber tube

Canister Purge Control Solenoid Valve

Blocked or bent tube to the MAP/BARO Switch Solenoid Valve

Cracked EVAP Canister

Absolute Pressure Sensor

MAP/BARO Switch Solenoid Valve



In replacing this valve they fixed the condition that sets this code, since it does not come back. They obviously did not find a leak or a blockage if they needed the valve. There are a lot of possibilities for this code but you can narrow it down with a few simple tests. If they replaced the Canister Purge Control Valve, it may be possible that the ECM grounds were still damaged at this time but that you may have had two problems at once (the Canister Purge Control Valve and the Canister Purge Control Solenoid Valve). Diagnosing an internal ECM problem can be very tricky and hard at times, since they can seem to work at times and not at others. Also, this code is very broad in its possible problems, and so if they did find a problem, they may not have been looking for a second underlying problem. Either way, this code does not return so we will not focus on it further.


Now I will explain the P0433 code and hopefully this will help you see the difference or possibly the connection between the two codes. You will see that this code is much more specific in the area of diagnosis. Remember, I am explaining this from my perspective on the information at hand and my experience with these types of problems.


P0433- EVAP Canister Purge Control Valve and Solenoid Valve Circuit

This malfunction is detected when one of two things happens. Type A Malfunction is detected when an improper voltage signal is sent to the ECM through the Solenoid Valve circuit, and Type B malfunction is detected when the EVAP Canister Purge Control Valve does not operate properly and is stuck open. Keep in mind the Canister Purge Control Valve is a mechanical, vacuum operated valve, NOT electrically operated.


Possible Causes:

For Malfunction Type A

-Harness or Connectors (The Solenoid Valve circuit is open or shorted)

-EVAP Canister Purge Control Solenoid Valve

- - - - - - - - - - - - - - - - - - - OR - - - - - - - - - - - - -

For Malfunction Type B

-EVAP Canister Purge Control Valve

-EVAP Canister Purge Control Solenoid Valve

-Vacuum Hoses clogged or disconnected

-EVAP control system Pressure Sensor


Malfunction Type A is detected as soon as the key is in the ON position. Malfunction Type B is detected only when test driven or when running the vehicle in the air. This is simply used in the diagnosis process. Since the two Nissan shops have come to the same conclusion that the ECM is not grounding properly, they have probably detected a Type A problem, with the Canister Purge Control Solenoid Valve.


Now, this is where I believe that the shop had you prepay and replace Canister Purge Solenoid Control Valve, believing that the solenoid/valve was internally faulted. The solenoid/valve can be tested by a simple bench test of supplying power and ground to its terminals and checking if air can be blown between the two passages. Either they did not test it in this manner or the solenoid/valve actually tested bad and so it was replaced. The simplest and fastest fix (and usually more logical) is to simply replace this valve because it may be intermittently sticking open with age or dust/debris internally. This is the most common occurrence. But, upon replacement of this solenoid/valve the code still returned.


They have at this point concluded the Canister Purge Control Solenoid Valve is not at fault but is not operating properly. The only possibilities left are a problem in the harness or connector to the solenoid/valve, or the ECM not supplying the proper power and ground to it. This can be checked simply by looking for power at the solenoid/valve, and checking continuity of the harness from the solenoid/valve to the ECM. The ECM will only apply ground when certain conditions (that are programmed into the ECM) are met. It will NOT apply ground when any of the following occur:

-Ignition is "ON" and engine is stopped

-Closed throttle is detected

-Low or High engine coolant temp is detected

-During deceleration

-Low vehicle speed (M/T models)


At this point the shop has detected power to the solenoid/valve but has not found ground. They should have also done a continuity check of the circuit, between the solenoid/valve and the ECM, to ensure the harness is ok and not broken, as well as a pin fit test on the connectors at the ECM and the Canister Purge Control Solenoid Valve to ensure they are not damaged. Thus, making sure that the communication between the ECM and the solenoid/valve is ok.


If, at this point they have concluded that there are no harness problems, or connector problems, they should use the Nissan Consult to activate the Canister Purge Control Solenoid Valve and look for the ECM to provide a ground. The Consult diagnosis computer can tell the ECM to activate the solenoid/valve upon command of the technician. Assuming that they have done all of this and they still do not receive ground to the Canister Purge Control Solenoid Valve, the conclusion is that the ECM is internally faulted and needs to be replaced.


OK! I think that is explained as best I can. I hope you understand that ECM failures are not very common but do happen sometimes, and that for a technician they can demand a lot of skill and patience to figure out. Sometimes, it is good to get a second opinion on something like this because it is not an everyday occurrence. I would suggest possibly seeing if you can get a used ECM, as they can be very pricey. Hopefully this answers your question and I hope you enjoy your Nissan overall. If there is anything else, just let me know. Thanks