2003 Chevy Tahoe P0120 and P2135 Fault: Reduced Engine Power After Throttle Body and TPS Replacement

A scan tool is essential to interpret the actual voltage readings from TP Sensor 1 as detected by the PCM. P0120 occurs when TP Sensor 1 voltage drops below 0.325 volts or exceeds 4.75 volts for more than one second—likely a high-voltage reading in this case. P2135 indicates that the voltage difference between TP Sensor 1 and TP Sensor 2 exceeds a set threshold for over two seconds, suggesting a mismatch or signal degradation.

The abnormal behavior appears to occur only during operation—especially when the engine is warm. The issue does not manifest consistently; it often resolves after shutting off the vehicle, removing the gas cap, and restarting. This intermittent pattern suggests a possible vapor lock or minor leak in the fuel system, though no visible leaks are present.

Check the 'freeze frame' data stored in the PCM—it may contain critical information about conditions when the fault occurred (e.g., engine temperature, load, RPM). Also review any EVAP-related codes or symptoms like difficulty refueling, erratic fuel gauge, or no fuel pressure. Consider testing the fuel cap’s sealing performance using a scan tool to verify if it's on the same 5V reference circuit as other sensors.

No additional diagnostic codes are present. The gas cap issue is particularly puzzling—only occurs when driving, and requires full disconnection and reseating of the cap to reset. I have an Actron AutoScanner and plan to test TP sensor voltages tomorrow. This intermittent behavior makes it difficult to diagnose without live data monitoring.

The gas cap issue is worth investigating further. Is it an OEM or aftermarket cap? Does the problem occur on every trip, short or long? When opening the cap, do you hear air rushing in or out? Has there been any work done at the rear of the vehicle (e.g., fuel pump replacement)? What happens if the gas cap is not fully tightened—could this create a pressure leak?

The gas cap is aftermarket, not OEM. My wife reports no audible air release when opening it. The issue can occur at any time during driving but seems to appear more frequently after the engine reaches operating temperature. No fuel pump replacement has been done. I will try leaving the cap slightly loose to see if a pressure leak triggers the fault.

I suspect a faulty Accelerator Pedal Position (APP) sensor. The correct resistance values should be approximately 1.35 ohms when the pedal is not pressed and between 3.00–3.30 ohms when fully depressed, with smooth movement during pedal input. My test showed 1.45 ohms unpressed and 4.70 ohms pressed—abnormal and jerky response. I replaced the sensor, but it did not resolve the issue. I will monitor further and update if results change.

Replacing the APP sensor did not fix the problem. After installation, the new sensor showed correct resistance values, but a day later the 'Reduced Engine Power' warning returned. Testing with the gas cap loose had no effect—same issue persists. I’ve now scheduled an appointment with a dealer for diagnosis, though I’m concerned about the cost (estimated $150) and whether they’ll identify the true root cause.

This solution applies to P2135—Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation: 1. Disconnect the battery terminal (+). 2. Reset the ECU (Engine Control Unit) by turning off the ignition and allowing it to power down. 3. Reconnect the battery. 4. Insert the key into the ignition but do not start the engine. 5. Fully press the accelerator pedal to the floor, then start the vehicle. Once running smoothly, release the pedal—the P2135 code should clear and the system will relearn properly. This method works for similar codes such as P3135. —Marc (Netherlands)

The issue was resolved at a second dealership. Upon replacing the throttle body wiring harness, there is a critical second step that must be completed to restore proper voltage and signal integrity. The first dealer failed to perform this step and used a spliced pigtail instead of a full harness—this resulted in low voltage to the throttle body. The second dealer correctly performed both steps and restored normal operation.

Could you please clarify what the 'second step' is when replacing the throttle body wiring harness? This would be extremely helpful for others experiencing similar issues with their 2003 Chevy Tahoe and P2135 code.

What exactly is the second step in the throttle body wiring harness replacement procedure?

I am experiencing the same issue—after replacing the throttle body, my vehicle enters 'Reduced Engine Power' mode with P2135. I’ve taken it to the same mechanic eight times in the past week; they relearn the codes each time but the problem persists. If there is a known second step required after wiring harness replacement, that would be invaluable for troubleshooting this common GM issue.

It's best to take the vehicle to a certified technician with expertise in diagnostics and electrical systems. This issue can be easily diagnosed using a digital multimeter (DMM), a scan tool capable of reading live data PIDs, and an understanding of system operation. GM has known issues with throttle body wiring connectors and grounding—performing circuit checks by wiggling the connector while monitoring voltage at the TPS sensors is recommended. Also verify that TP Sensor 1 and TP Sensor 2 readings agree during operation using a scan tool. The root cause is likely either a wiring or ground fault, as GM issued a technical service bulletin (TSB) addressing this specific problem.