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1999 Blazer 4x4 P0740 P0745 and P1860 Codes: Causes and Fixes After 4WD Use

Model: 99 Blazer Fault Code: P0740 Posted: 2010-02-07 15:38

I own a 1999 Blazer 4x4, and today while using 4WD to back up onto my slippery driveway into the garage, the engine light came on. I used an OBD2 scanner and retrieved codes P0740, P0745, and P1860. Previously, after shopping, the vehicle ran normally with no issues. The transmission felt fine during operation. I cleared the codes, but due to severe weather, I can't take it out for a test drive yet. Given that my transmission was replaced about 18 months ago, what could be causing these errors? Are they related to the 4WD system or a sensor malfunction? Thanks for any insights.

Related fault codes

P0740P0745P1860

Comments (2)

Anonymous 2011-02-07 22:59

I'm not certain, but I recently had two vehicles with similar codes. My '98 Blazer stopped starting for two weeks—turns out it needed a new starter. This could be a solenoid or starter issue, possibly causing a circuit malfunction in the transmission or transfer case. Clear the codes and drive normally for a while to see if they return. Also check fuses—especially the 4WD fuse located on the driver's side dashboard and the relay under the hood. There may also be a 4WD-specific fuse or relay there. Check all components related to 4WD, transmission, and transfer case. I suspect it’s just a minor glitch. Good luck—I’ve found that in my '98 Blazer, most such codes resolve after driving for some time. If you were recently working on the vehicle or disconnected the battery, that might have disrupted the circuit signals. For me, clearing the battery and driving helped fix it. I hope this helps too.

Anonymous 2011-02-07 23:11

If the issue occurs during 4WD use, try these steps: 1) Disconnect the battery for at least one hour. 2) Locate the 4WD control module—usually under the dash, in the glove compartment, or behind the passenger side panel near the glove box. Remove it and disconnect for about an hour. Bring it inside to warm up slightly, gently tap it a few times, then re-install. Reconnect the battery afterward. This may reset the 4WD module and resolve faulty signals being sent to the transfer case or transmission. Did your 4WD function properly before? Does it work now? There’s a switch at the top of the transfer case and an electronic switch motor involved in 4WD operation, along with three vacuum hoses exiting the top of the transfer case that run throughout the vehicle—eventually connecting to an actuator under the battery tray. This actuator moves a plunger to engage 4WD. The entire system is complex and prone to issues. Also, there’s likely a switch on the front differential (I can’t recall the exact name right now). All these vacuum lines also affect heater functions—so it's quite intricate. Always check fuses and relays first. Disconnecting the battery before removing the 4WD module is critical. I believe this issue may stem from a software glitch or sensor misreading rather than actual hardware failure. In my experience with '98 Blazers, most such OBD codes are actually caused by temporary signal errors—often due to faulty sensors or modules that need time to reset. A simple disconnect and reboot often resolves them. Good luck!