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Gm anti theft

Printed From: Unofficial Allis
Category: Other Topics
Forum Name: Shops, Barns, Varmints, and Trucks
Forum Description: anything you want to talk about except politics
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=163317
Printed Date: 19 Jul 2025 at 1:11am
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Topic: Gm anti theft

Posted By: bobkyllo
Subject: Gm anti theft
Date Posted: 01 Aug 2019 at 9:51pm

Guys, I've got a 1998 Chevy Blazer in the shop. It was towed in with the belief it was the fuel pump. I hitched up the fuel pressure tester and I have 55 psi. That's plenty. Hit the key and it starts and dies right away. Security lights flashing. I put my code scanner on and I get code p1631. Looked up the code and it says the security code doesn't match the one in the computer.

Well if memory serves me the security light was on steady the last time I had it in the shop for service work.

What do you guys know about this and where should I go next with this repair. Do I just need to do a relearn of key on wait ten minutes turn key off and repeat for a total of 3 times. Might I need to actually look for a broken part?

Like I say I have just a code scanner so if you lead me down a path that requires a more in-depth diagnostic tool I'll have to pass on this job.

Memory also serves there was a bypass a person can do where you cut a wire and put in a resistor I think.

From what I can tell is there is no chip in the key.

Replies:

Posted By: Dakota Dave
Date Posted: 02 Aug 2019 at 12:18am

Try the relearn procedure. Don't cut the wire and put a resistor it dosent work for long. Order a bypass kit from EBay. I put one in 10 years ago works great. Had to cut two wires and wire it in it took me a couple hours to install. When Security checks the key it sends the correct code for the inquiry https://newrockies.com/passlock-bypass-2/" rel="nofollow - https://newrockies.com/passlock-bypass-2/ is what I installed

Posted By: Steve M Ind
Date Posted: 02 Aug 2019 at 8:30am

If it's the central port fuel injection which I believe it is then it needs 60 psi to start, then only about 45 psi to keep it running. Try pinching off the return line then turn the key to on and see what the pressure is then. If the pressure increases to 60 or over see if it will start. If pressure doesn't increase then I'm leaning towards a weak pump. Also security light should come on when key is turned on for a bulb check then go out after about 8 seconds. If light stays on or flashes then you have a security problem. Might try the 30 minute relearn procedure. If that fails then it looks like you'll need a new passlock sensor.

Posted By: Dakota Dave
Date Posted: 02 Aug 2019 at 3:05pm

Code 1631 is theft deterrent incorrect. Try relearn itll probably come bach amazon has both passlock sensors and bypass kits

Posted By: chaskaduo
Date Posted: 02 Aug 2019 at 4:10pm

Found this for ya.

http://www.silveradoss.com/forums/topic/50645-p1631-theft-deterrent-password-incorrect/" rel="nofollow - http://www.silveradoss.com/forums/topic/50645-p1631-theft-deterrent-password-incorrect/

HTH

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1938 B, 79 Dynamark 11/36 6spd, 95 Weed-Eater 16hp, 2010 Bolens 14hp

Posted By: bobkyllo
Date Posted: 02 Aug 2019 at 10:59pm

Good news guys, the vehicle runs again!!!! For now atleast.... I did the relearn and it runs. I'm farting rainbows ha ha ha. Anywho I'm a bit skeptical though. I Never fixed anything, why do you suppose it was giving me trouble?

I'm thinking if this gives the lady trouble again we will do a bypass. She didn't have alot of money so I did the cheapest thing first and we got lucky.

Thank you guys for your help.

Posted By: tractorboy
Date Posted: 04 Aug 2019 at 3:07am

We had a 98 monte carlo,after changing a few parts I gave up. Traded in cause mama has a one strike rule!!!

Posted By: Dakota Dave
Date Posted: 04 Aug 2019 at 8:35am

When my suburban started acting up no start security light flashing I did the relearn lasted a little while probably a couple weeks then it did it again relearn lasted a few days. Then relearn again to get home. By then the bypass module showed up took me a couple hours to install it. Read the instructions a few times finally cut the wire hooked it up in a few minutes did the relearn and it's worked for about 6 years.

Posted By: bobkyllo
Date Posted: 04 Aug 2019 at 11:28am

Well.... The relearn lasted all of 2 days. I guess we will be doing some more work on it.

Posted By: Brian Jasper co. Ia
Date Posted: 04 Aug 2019 at 6:27pm

Probably needs a new ignition lock. There's 2 brushes that contact the resistor in the key and 2 brushes that touch the tumbler to the housing. All those moving connections with a system that reads tenths of ohms, what could go wrong...

Read the resistance of the key pellet, then bypass the ignition switch circuit with a resistor that matches the key. Do the relearn and it should never be a problem again.

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"Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford

Posted By: Dakota Dave
Date Posted: 05 Aug 2019 at 10:00am

Theres no key pellet on pass lock system