ODBII Codes

I have a '96 Blazer that runs really rough. The first code it gave me was a bad egr valve so I replaced that. No improvement. Now it gives me P0122 (PPS/TPS A circuit low input) and P0440 (EVAP malfunction). I don't think changing the tps would be too difficult for a shadetree like me, but what about the evap problem? Any input from anyone?

Try using the search function on this forum to look for the codes you mention. I remember reading something about a loose gas cap causing a code for the evap system.

A loose gas cap is the most common cause for a P0440, just be aware that unless you use a scanner to clear the code, the PCM self test may take a week to turn the light out.

Next time you don't have to replace the EGR valve, just clean it and reinstall.

Well, now that I've determined that it's not the gas cap causing the P0440, can I assume there's a leak somewhere in the EVAP? Is that something I'm going to have to have checked out by a mechanic?

How did you determine it was not the gas cap?? what tests did you do??

Uh oh. Didn't expect that question. All I did was go out there and hand-tighten it. It didn't really feel loose, so I figured it was not the problem. Are there other tests I should be doing on the gas cap?

While we're on the subject of the P0440 code, I've replaced my gas cap. I've checked both solenoids (manifold and evap canister) for operation by removing them and checking for open/close with battery voltage while blowing through them to detect that they actually seal and unseal (I realize this isn't the best check for a good seal, but at least there are no gross defects). I've checked for good connections to both solenoids, and they appear to be OK. I've also looked for bad or disconnected hoses to and from the evap canister. When I clear the code, it usually stays gone for a few days at a time, and it usually comes back on right at startup as opposed to while driving. This week however, I was driving up my driveway, which is pretty steep, and after I stopped midway and then started back up it, the SES light came on... another P0440. This seems to happen more when my tank is less than half full too, if that helps anyone. Any advice?

While we're on the subject of the P0440 code, I've replaced my gas cap. I've checked both solenoids (manifold and evap canister) for operation by removing them and checking for open/close with battery voltage while blowing through them to detect that they actually seal and unseal (I realize this isn't the best check for a good seal, but at least there are no gross defects). I've checked for good connections to both solenoids, and they appear to be OK. I've also looked for bad or disconnected hoses to and from the evap canister. When I clear the code, it usually stays gone for a few days at a time, and it usually comes back on right at startup as opposed to while driving. This week however, I was driving up my driveway, which is pretty steep, and after I stopped midway and then started back up it, the SES light came on... another P0440. This seems to happen more when my tank is less than half full too, if that helps anyone. Any advice?


What year, make and model do you have?

Usually when the light comes on at start up, it's an electrical concern. And it comes on after the second time the PCM sees the failure.

Besides the two valves, there is also a vacuum sensor on top of the fuel tank.