The quickest way to reset the PCM (engine control module) on a 1999 Jeep Cherokee is to cut power to the computer and then restore it, which clears learned memory. In practice, you typically disconnect the battery, wait a short period, and reconnect in the correct order. You can also clear diagnostic codes with an OBD-II scanner if needed. This guide walks you through safe steps and what to expect afterward.
Why you might reset the PCM
Resetting the PCM clears adaptive memory (like fuel trims and idle learning) and can help with minor drivability issues after maintenance. It does not fix mechanical problems by itself, and any persistent fault codes may reappear if the root cause remains unresolved.
Manual reset: battery disconnect method
Use this method to perform a full power-down of the engine computer. The steps below assume you are working safely and with the ignition off.
- Park on a level surface, apply the parking brake, and remove the key from the ignition.
- Open the hood and use a wrench to loosen the negative battery terminal clamp, then lift the negative cable off the terminal.
- Optional but recommended: loosen and disconnect the positive terminal as well to ensure a complete power-down.
- Wait 15 to 30 minutes to let the PCM power down and all capacitors discharge.
- Reconnect the positive terminal first, then reconnect the negative terminal, and tighten the clamps securely.
- Replace any covers, close the hood, and start the engine. If your radio prompts for a code, enter the correct code. Drive normally to allow the PCM to relearn.
This manual reset clears the PCM’s learned data and resets readiness monitors. If a fault persists or a warning light remains, further diagnosis may be necessary rather than a simple reset.
Alternative: Clearing diagnostic codes with an OBD-II scanner
If you want to erase stored trouble codes without a full power-down, an OBD-II scanner provides a controlled way to clear codes. This method is helpful after repairing a detected fault or if the check-engine light is on but you want to reset the codes to verify the repair. Note that codes can return if the underlying issue isn’t fixed, and some monitors may require driving to complete.
- Locate and plug an OBD-II scanner into the vehicle’s OBD-II port under the dashboard near the steering column.
- Turn the ignition to the ON position with the engine off and allow the scanner to communicate with the ECU.
- Use the scanner’s menu to read current and pending codes, then select the option to Clear Codes or Erase Codes.
- Turn off the ignition, start the engine, and drive normally to allow the onboard monitors to run and recheck for codes.
Clearing codes with a scanner is a diagnostic step and not a substitute for addressing root causes. If the check-engine light comes back on, you’ll need to retest the system to identify and repair the problem.
Drive cycle considerations after reset
After resetting the PCM, a drive cycle helps the computer relearn fuel trims, idle behavior, and timing maps. Some drivers notice a rough idle or temporary quirks until the system relearns. Regular driving—covering city and highway conditions—usually completes the relearn over a few cycles.
Recommended drive cycle
For best results after a reset, perform a normal drive cycle: short trips to reach operating temperature, followed by cruising at steady speeds, and a few gentle accelerations and decelerations. Avoid heavy loads or aggressive driving until the PCM has completed its relearn process.
What to expect immediately after a reset
You may notice minor changes in idle behavior or throttle response as the PCM recalibrates. If any fault codes were cleared, the check-engine light may stay off for a while unless another issue triggers it again. If problems persist, a more thorough diagnostic may be required, possibly with a professional scan tool.
Summary
Resetting the PCM on a 1999 Jeep Cherokee is commonly done by powering the computer down and back up: disconnect the negative battery terminal, wait 15–30 minutes, and reconnect in the proper order (positive first, then negative). For clearing stored codes, an OBD-II scanner offers a controlled alternative. After any reset, drive the vehicle normally to allow the PCM to relearn and complete its readiness monitors. If a warning light remains or returns, pursue targeted diagnostics to address the underlying issue.