* Low Transmission Fluid: This is the most common and easiest to check first. Low fluid can cause harsh shifts and slipping, leading to the thump you describe as the transmission struggles to engage 3rd gear. Check the fluid level and condition. Dirty or burnt fluid also indicates a bigger problem.
* Worn or Damaged 3-2 Shift Solenoid: This solenoid controls the shifting between 2nd and 3rd gear. If it's malfunctioning or worn, it could cause a harsh engagement or even skip the gear altogether. This is a relatively common failure point in automatic transmissions.
* Transmission Valve Body Problems: The valve body houses the solenoids and other valves that control the transmission's operation. Internal wear, blockage, or a faulty valve within the valve body could cause erratic shifting. This is a more involved and expensive repair.
* Worn Clutch Packs or Bands: The internal clutch packs and bands wear over time. If these are worn, they may not engage fully, leading to a harsh engagement or slipping, especially under load at slow speeds. This is a major transmission repair.
* Torque Converter Issues: While less likely to be the *sole* cause of a thump specifically into 3rd, a failing torque converter can contribute to harsh shifting throughout the transmission.
* Low Engine RPM: If you're trying to accelerate in a very low gear (1st or 2nd) at extremely low RPMs, the transmission might be forced to shift into 3rd gear abruptly, causing the thump. This is less likely a mechanical issue and more a driving style issue.
What to do:
1. Check the Transmission Fluid: This is the absolute first step. Check the level and condition of the fluid. If it's low, add the correct type (consult your owner's manual). If it's dirty or burnt, a complete transmission fluid and filter change might help, but a deeper problem is likely.
2. Have it Diagnosed: Take your Catera to a qualified mechanic specializing in automatic transmissions. They can perform a diagnostic scan to check for any trouble codes stored in the transmission control module (TCM). This will help pinpoint the problem more accurately.
Ignoring the problem will likely lead to more severe transmission damage and a much more expensive repair down the line. It's best to address it promptly.
