Reset the SES light?

I had some codes come up that i fixed. one was P1404, i switched out the EGR valve. the other was P0128(i think) i changed the spark plugs and wires, and the other was a coolant temp low, or something like that. So i switched out the thermostat. but the light is still on and took it to autozome and the same codes came up. so i was just wondering if there is a way to reset the SES light to see if i really did fix the problems. thanks.

Tommy.

Take off the positive battery cable for about 5 minutes, that should clear the codes. Also check the grounding on the temp sensor, that can cause low temp problems also. That is if you get the same error code for low temp.

How do i check the grouding on the sensor?

All vehicles 1996 and newer are equipped with On Board Diagnostics second generation, (OBDII). Disconnecting the battery on these vehicles will not reset the OBDII system. There are three different classifications of Diagnostic Trouble Codes, (DTC’s) disconnecting the battery may or may not turn the Service Engine Soon, (SES) light off. Disconnecting the battery will not remove any DTC's, data in the history file, or reset the IM readiness monitors. The DTC’s, history data file, and IM readiness monitor status is burned into the OBDII memory as it occurs. If the battery goes totally dead for whatever reason, (disconnecting the battery would simulate this), DTC’s, data in the history file, and IM Readiness status will still be available to the technician, (for diagnostic purposes), when power is restored to the system. This allows the technician to access all data concerning how the vehicle has been performing since the last time the system reset. Most code readers are not capable of resetting the system. The ONLY way to reset OBDII, including DTC’s, data in the history file, and IM readiness monitors is with a scan tool.

Thanks for that info, but the whole reason i want to clear and see if i did indeed fix the problems is that my vehicle didnt pass DEQ. So i want to make sure that the problems are corrected before i take it back in, and if the light is still on, i want to make sure that it doesnt just show the old problem and not be adjusted to the new fixes that were made.

If all the issues were fixed -- the light should go out by itself with in three drive cycles. If it doesn't then there are still issues that have to be resolved before the PCM is satisfied and turns off the SES. You can get a code reader that will reset the PCM and turn off the light from Wal-Mart for $100 bucks. The big BUT to a DiY PCM reset, it can take a few drive cycles before the PCM is completely satisfied with efficiency of the smog system. So the light could come back on right away or with in a couple days depending on how you drive it indicating the issue really wasn't fixed.
The Master is right about the PCM storing codes. The way the OBDII works from what I understand, the software in the PCM is designed to constantly compare inputs from the sensors (i.e. continually tests) to the hard coded data base in the computer and then make adjustments to timing, fuel, etc. When the computer detects a sensor(s) reading out of limits it turns on the SES light. Depending on the sensor(s), the computer will not turn off the SES light until it has seen enough comparisons that are in the limits of the data base and/or drive cycles in order to satisfy requirements that are engineered to meet smog regulations.

The first thing they look at is the SES light, if it's on, they will not continue with the inspection. If it's off, they will check the IM Readiness status. If all monitors are not complete, they will not continue the inspection until they are. Have the system reset, drive it until the IM readiness monitors are complete, if the light comes on, there's a problem.

Actually there is a short-cut PCM reset procedure that has been floating around various GM "F" and "W" body car forums including AF for years where an odb-ii scanner is not required. Confirmed to clear all types DTCs (A, B, and C), history codes, reset IM readiness, clear TAPS (adaptive shift) and puts the PCM into a re-learn mode. No guarantee it will work on all '96 and up Grand Prixs or other GM cars/trucks but may not hurt to try it.

If it doesn't work then you will need a odb-ii scanner. If it works don't ask me how or why it does. :grinno:

Several years ago when I did not have my odb-ii scan tool I used the procedure to reset CAT convertor related DTCs and the SES/CEL light. Miraculously, it worked.


Make sure ignition sitch is OFF. Remove battery POSITIVE cable.

Take ignition to full on/start position and hold for minimum period of 1 minute.

Take ignition back to off. However, key will not return to off and will be stuck due to lack of battery power.

Reconnect battery positive. Various relays will chatter.

Now take key forward then back to off. It should now remove.

Re-insert key and start engine. Note: SES/CEL should be off unless a Class "A" DTC error code immediately triggers it.