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Chevrolet Silverado 1999-2006: Transmission Shifting Problems and Diagnostics

Take a couple of easy steps to see why your truck is not shifting. No need to panic yet.

By Mark Garrett - June 23, 2015

This article applies to the Chevrolet Silverado GMT800 (1999-2006).

Your transmission is just supposed to shift and leave you alone. When it fails to do so, you naturally get upset either because of the trouble you are not looking for or the fear of how much the fix may cost. Before your day turns grim, let's take a look at some things that may be going on. Don't tense up too much just yet. Below are several common things to check.

Materials Needed

  • Electrical tester or multimeter
  • Drain pan
  • 10mm socket
  • Ratchet and extensions
  • Dexron VI transmission fluid
  • Transmission filter kit

Step 1 – Check fuses and wiring

  • If any accessories have been added like a remote start or aftermarket sound equipment, the wiring may have been damaged or compromised.
  • Sometimes, you have to remove them.
  • Gauges that are not working could mean a faulty ignition switch. This has been an issue on this truck series. A bad ignition means that there may not be full voltage available to activate the shift solenoids. Replacing the switch will cure the problem.
Figure 1. Look for blown fuses or damaged wiring.

If the wiring and fuses are good, go to Step 2.

Step 2 – Get trouble codes from PCM

The transmission in your truck is controlled by the PCM (powertrain control module). It is sophisticated enough to be able to tell you if there is an issue with your transmission. The local auto supply may be able to scan it, but it may take a better scanner to dive deeper into the PCM. Any code found can help you pinpoint your problem.

  • A code may pinpoint an exact solenoid that needs replaced or be more general.
  • A shifting solenoid is pretty easy to replace; although, you have to remove the transmission pan to change them.
  • A general trouble code may show that the transmission is slipping, but not pinpoint what the issue may be.
Figure 2. Look for check engine lights.

(Related Article: How to Use OBD Scan Tool - ChevroletForum.com)

If there are no trouble codes, proceed to Step 3.

Step 3 – Check transmission fluid level

Your transmission depends on fluid to operate. If your transmission fluid is low, the transmission may not be able to shift correctly. Plus, you can see and smell the fluid to tell if it is burnt.

  • Add fluid to the correct level if low.
Figure 3. Check your transmission fluid dipstick.

If fluid level is acceptable, move on to Step 5. If fluid is burnt proceed to Step 4.

Step 4 – Replace transmission fluid and filter

Clean, fresh transmission fluid is the life blood of your transmission.

  • When you remove the transmission pan, you can see if there is any junk in it that may be coming from the clutches or internal parts.
  • Some wear is normal and even if there is stuff there, it may not mean that failure is imminent.
  • If the contents of the pan look like a train wreck, a rebuild is probably in order
Figure 4. A good looking, reconditioned pan.

If changing the transmission fluid does not correct your issue, go to Step 5.

(Related Article: How to Replace Transmission Pan and Change Fluid - ChevroletForum.com)

Step 5 – Commonly reported issues

Reverse slips are slow to engage or is out:

  • Low-reverse clutches are worn out - rebuild transmission
  • Broken sunshell - rebuild transmission
  • Worn out boost valve - replace

1-2 shift does not happen at full throttle, there is a shudder when shifting, delayed or harsh shift:

  • Possible bad throttle position sensor - replace
  • Issue with the second gear apply circuit, service servo assembly, or 1-2 accumulator - pressure test
  • 1-2 accumulator piston issue - replace

No 3rd or 4th gear:

  • 3-4 clutches are worn out - a transmission rebuild is required

No Reverse in 2nd or 4th gear, 1st and 3rd only:

  • Sunshell is broken (do not drive vehicle) - a transmission rebuild is required

No 2nd or 4th:

  • 2-4 band is slipping, worn out - a transmission rebuild is required

Transmission goes into gear harshly, vehicle sluggish from start, no 1st, 4th, or torque convertor lockup, manual 2nd, 3rd, reverse gears only working, check engine light is on:

Transmission is in Limp Home Mode or has no electrical power. Also, PCM has lots of error codes.

  • Check underhood transmission fuse - replace
  • If fuse burns out again, check circuit for short - repair shorts
  • Check for PCM code for fault causing Limp Home Mode

No 1st or 4th, shift 2nd to 3rd by itself when put in D or OD and locks converter:

Goes into gear but sluggish like brakes are on, can manually shift to 2nd to get moving, then put in OD and truck shift 3rd as well as 4th are OK. Also, torque convertor locks up.

  • Shift solenoid A or failed or wiring issue between PCM and Transmission - needs diagnostic testing

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