Fix P0122: Mechanic’s solutions that WORKED

The code P0122: Throttle Position Sensor/Switch “A” Circuit Low Input signifies an issue with the Throttle Position Sensor (TPS), specifically indicating that the sensor’s output is unexpectedly low. This fault can arise from various factors, ranging from wiring issues to the sensor itself. It is crucial to address this code promptly, as it can impact the vehicle’s performance and fuel efficiency.

P0122 quick overview

Meaning	P0122: Throttle Position Sensor/Switch “A” Circuit Low Input
Is it serious?	It can be. This code often affects drivability and can lead to increased emissions, poor fuel economy, and engine performance issues.
Possible causes	– Faulty or misaligned throttle position sensor – Corroded or damaged TPS connector – Open or short in the TPS signal wire – Faulty Engine Control Module (ECM), though less common – Issues resulting from recent repairs or engine replacements
How to diagnose?	Check TPS voltage: Ensure it receives the correct reference voltage (usually 5V). Inspect connector: Look for damage or corrosion at the TPS connector. Wiggle test: Manipulate wiring while monitoring TPS voltage for intermittent faults. Review repair history: Consider any recent engine work or replacements. Visual inspection: Thoroughly inspect for foreign objects or minor damages. Run new wires if needed: As a workaround for persistent wiring issues.

Imagine the throttle position sensor as the car’s way of knowing how far you’re pressing the gas pedal. When the code P0122 pops up, it’s like the car is saying, “I’m not getting a clear signal from the sensor!” This could mean the sensor is faulty, the wires are damaged, or there’s a problem in the connections. It’s like trying to listen to your favorite radio station but only getting static. It’s definitely something you’ll want to fix to keep your car running smoothly.

How to fix P0122 – learn from mechanics

I looked into P0122 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Honda

Vehicle Information

• 2002 Honda Civic LX 1.7L

Issue + Repair History

• MIL on with code P0122 (throttle sensor low).
• Tested at sensor harness and PCM harness connector, cause not verified.
• Throttle sensor comes with the throttle body, costing over $500.

Other OBD2 Codes + Test Results

• N/A

Suggestions to Fix Issue

• Replace the throttle body as TPS is riveted onto it.
• Use a graphing multimeter or an analog meter to diagnose.
• Consider replacing with a used TPS from an older model to save costs.

Final Fix

• Customer has not returned, but reported that the vehicle is working okay now after throttle body replacement.

Case #2: Saturn

Vehicle Information

• 1998 Saturn SC2 1.9L

Issue + Repair History

• Car was towed in with a no-start condition. At the shop, it started and ran okay initially, then ran rough on the next start.
• Codes P0122 (TPS sensor input low) and P1635 (No 5-volt reference) were found.
• Sometimes after clearing codes, the engine runs well; other times it runs rough.
• When running rough, TPS input voltage drops to 0.5 V.
• Suspected voltage spike causing computer to lose 5-volt reference.

Final Fix

• Discovered a staple from a tin foil insulator for a boom box had pierced the gray wire for the gas tank pressure sensor in the rear, causing a short.
• After disconnecting the connector in the left kick panel that goes to the back of the car, the 5-volt reference returned.

Other OBD2 Codes + Test Results

• P0122: TPS input low.
• P1635: No 5-volt reference.

Suggestions to Fix Issue

• Check for short or intermittent short in primary sensors, especially the TPS.
• Scope the TPS.
• Check TPS wire harness for chafing on intake.
• Look for chafed wiring in the engine compartment.
• In the left footwell of the driver’s area, check the grey wire to the connector for chafing on the frame of the car.

Additional Notes

• In 1998 Saturn models, P0122 can sometimes cause an intermittent high idle (1600 RPM) but is not related to rough idle and no-start conditions.

Case #3: Chevrolet

Vehicle Information

• 2002 Chevrolet Astro 4.3L

Issue + Repair History

• Intermittent no-start condition.
• No fuel pressure when failing to start, but crank signal present.
• Antitheft system likely not disabling the fuel pump, as it showed a flash of voltage then steady power after multiple key turns, leading to the van starting.
• Fuel pump was replaced a few months back.

Final Fix

• Found corrosion on the back side of the connector pins at the fuel pump.
• Replaced the pins, and the van didn’t exhibit the no-start issue again.

Computer Codes & Descriptions

• P0341B: Camshaft position sensor performance (Current Code)
• P0341: History Code
• P1122: History Code
• P0122B: TP sensor circuit low performance (fail since clear)
• P0300: Engine misfire detected (fail since clear)
• P1122C: TP sensor circuit intermittent low voltage (fail since clear)

Other Test Results

• Camshaft sensor: Good
• Fuel pump: Good

Suggestions to Fix Issue

• Address DTCs P0122B and P1122C, as they may disable the pump.
• Swap the fuel pump relay with another one with the same part number.
• Address all DTCs first, then troubleshoot the intermittent issue if it persists.
• Check if the fuel pump is aftermarket or OEM (ACDelco); aftermarket pumps can be problematic.
• Consider the possibility of an ignition switch failure, especially if the vehicle is primarily used in town.

Case #4: Subaru

Vehicle Information

• 2000 Subaru Legacy L 2.5L

Issue

• Persistent P0122 code indicating “Throttle Position Sensor/Switch ‘A’ Circuit Low”.
• Poor idle quality.
• Issue began after an engine swap with a used engine.

Repair History

• Replaced the Throttle Position Sensor (TPS) with an updated unit from Subaru as per a TSB (Technical Service Bulletin).

Final Fix

• A new signal wire was run from the ECM to the TPS to bypass potential issues in the existing wiring, allowing the car to pass inspection. This was deemed a suitable solution given the vehicle’s high mileage and the customer’s desire to avoid high repair costs.

Computer Codes & Descriptions

• P0122: Throttle Position Sensor/Switch ‘A’ Circuit Low

Other Test Results

• TPS voltage: 0.4V closed, 3.9V WOT (Wide Open Throttle)

Suggestions to Fix Issue

• Clean the throttle body, as this has resolved idle issues in many vehicles.
• Verify reference voltage at ECM and at the sensor. The expected TPS specs are 0.3V-0.7V closed and 4.3V-4.9V WOT, which suggests the readings are below spec.
• The TSB for TPS concerns may only apply to Legacy Outback models with an H-6 engine.
• Check freeze frame data for TPS readings to understand when the low voltage is occurring.
• Inspect wiring and connections, particularly for the TPS signal and 5V reference, considering the possibility of an open or short to ground.
• Overhaul engine grounds, ensuring they are clean and tight.
• Examine the TPS connector and harness closely for any damage, especially given the issue started post engine swap.
• The poor idle quality may not be related to the P0122 code; check valve timing and Mass Air Flow (MAF) data.
• Pay close attention to harness connectors and the harness on the intake manifold for any chafing or damage.

Additional Notes

• It’s noted that the issue may not directly be related to the P0122 code and that additional factors such as valve timing and MAF data should be considered in diagnosing the poor idle quality.

Case #5: GMC

Vehicle Information

• 1999 GMC Savana 3500 5.7L

Issue

• MIL (Malfunction Indicator Lamp) on with a P0122 code indicating “TPS Voltage Low”.
• Lab scope shows reference voltage at 5V, KOEO (Key On Engine Off) TPS signal output at 0.69V with a perfect TPS pattern.
• Alldata states that the code is set when the ECM sees less than 0.25V on the TPS signal.

Repair History

• TPS (Throttle Position Sensor) replaced by another shop.

Final Fix

• A thorough visual inspection revealed a thin slice of metal in the TPS at the connector, intermittently shorting the signal out to ground. Removing the offending metal resolved the issue.

Computer Codes & Descriptions

• P0122: TPS, Low Voltage

Suggestions to Fix Issue

• Verify what voltage the scanner reads for the TPS.
• Check for an intermittent short to ground on either the signal return (most likely) or the reference. Pay close attention to areas around components that get hot or have sharp edges for signs of wire chafing.

Additional Notes

• The case underscores the importance of thorough visual inspections in diagnostics, as even minor intrusions like a small slice of metal can cause intermittent and misleading electrical faults.
• Persistence and attention to detail were key in resolving an issue that could easily be misdiagnosed as a faulty component or more complex electrical problem.

Advice	Brief Description of Diagnostic Step
Check for proper voltage and ground at the TPS	Ensure that the TPS is receiving the correct reference voltage (typically 5V) and has a good ground connection.
Inspect TPS connector and wiring	Look for any signs of corrosion, damage, or loose connections at the TPS connector. Hand-trace the wiring for potential chafing or damage.
Perform a wiggle test	While monitoring TPS voltage, manipulate the wiring harness and connectors to check for intermittent faults or shorts.
Consider past repairs and related history	Take into account any recent repairs or changes, such as an engine replacement, that might have affected the TPS circuit or its wiring.
Visual inspection is crucial	A thorough visual inspection can reveal minor yet critical issues such as foreign objects or slight damages that could cause intermittent faults.
Swap with known good components (if applicable)	In cases where a relay or similar component is involved, swap it with a known good one to rule out component failure.
Check and clean the throttle body (if relevant)	Dirt or carbon buildup in the throttle body can affect TPS readings and performance. Cleaning may resolve issues related to idle quality.
Run new wires if necessary	If wiring issues are suspected and cannot be resolved, running new wires from the ECM to the TPS may be a workaround, especially in older vehicles where a complete repair isn’t cost-effective.