P0120 — Throttle Position Sensor (TPS) A Circuit

OBD-II diagnostic trouble code reference.

P0120 is an OBD-II diagnostic trouble code defined under SAE J2012 — the standard governing fault-code reporting on light- and medium-duty vehicles since 1996. TPS A circuit malfunction detected The fault sits in the engine system, so the typical service path is targeted inspection of that subsystem before broader troubleshooting. This page summarizes what P0120 means in plain English, common causes when documented, the standard 5-step diagnostic flow, and any FMCSA roadside-inspection violations that cite this code.

Code:: P0120
System:: OBD-II
Category:: engine
Severity:: High severity

Severity classification

P0120 is classified as High severity.

Meaning

What this fault code indicates.

TPS A circuit malfunction detected

Typical Symptoms

Driver-observable indications commonly associated with this code.

erratic idle
poor acceleration
limp mode

Related Codes

Codes commonly diagnosed alongside this one or sharing the same code family.

P0121 P0122 P0123 P0124

How to diagnose P0120

Confirm the code is current, not historical. Use a scan tool to read both Active and Historical (or Inactive / Logged) codes. P0120 matters most when it appears in the Active list. Historical-only codes can reflect a one-off event that has already cleared.
Note any other codes set at the same time. Pull the full list of codes the ECM is reporting and write down which ones share a Freeze Frame timestamp with P0120. A simultaneous failure pattern often points to the root cause faster than any single code in isolation.
Check the most-common cause for this code. Before replacing parts, check the most-common cause for this code (start with the simplest root causes — wiring, connector corrosion, sensor failure — before replacing larger components). The "Common causes" panel on this page lists the established root causes when we have published data — when it is empty, fall back to the OEM service manual or a licensed diagnostic database.
Inspect the relevant component. Physically inspect the engine subsystem: connector pins, wiring harness, ground straps, and sensor body for damage, corrosion, or contamination. Many DTCs are wiring or connector faults rather than failed components.
If unsure, take to a qualified diagnostic technician. If the code returns after repair, or if the actual fault is not obvious from the inspection, escalate to a shop with the proper licensed scan tool for the vehicle (heavy-duty trucks need a J1939-aware tool, not a generic OBD-II reader). Replacing parts based on the code alone is the most common cause of repeat repair costs.

Frequently asked questions about P0120

What does P0120 mean? ▾

P0120 (Throttle Position Sensor (TPS) A Circuit) — TPS A circuit malfunction detected

Is P0120 a serious problem? ▾

P0120 is classified as a higher-severity code (High severity). It usually signals a condition that warrants prompt attention — continuing to drive the vehicle without diagnosis can lead to collateral damage or failure of related components.

Can I drive with P0120 active? ▾

Driving with an active P0120 is not recommended. Higher-severity codes can indicate conditions where continued operation risks further damage or vehicle failure. If the code is set, plan to get the vehicle to a service location at the next safe stop.

What's the typical cost to fix P0120? ▾

We don't have public cost data on this for P0120. Repair cost depends heavily on the actual root cause (which the code itself does not specify), the vehicle make and model, and labor rates in your area. Get a diagnostic charge quote from a shop that has the proper scan tool for the vehicle before authorizing any parts replacement.

Will P0120 cause an emissions failure? ▾

P0120 is not in the standard set of emissions monitors. By itself it should not cause an emissions test failure, but the underlying fault may. We don't have a definitive emissions-impact classification on file for P0120.

How is P0120 different from related codes? ▾

P0120 sits in the same family as P0121, P0122, P0123, and 1 more. They share a common system or component, but each identifies a distinct failure mode or location. The "Related Codes" panel above lists every sibling we have on file so you can compare meanings side-by-side.

Where can I find the official spec for P0120? ▾

OBD-II codes are defined under SAE J2012, which is a paid standard published by SAE International. The full canonical text is available through SAE's Digital Annex (www.sae.org/standards/content/j2012/) or licensed scan-tool databases (Mitchell, Alldata, Identifix). We don't republish the licensed text here — the description above is a plain-English summary.

What inspection violations might cite P0120? ▾

We don't have any FMCSA roadside-inspection violations on record that cite P0120 directly. Most inspection violations reference FMCSR sections (49 CFR 393.x, 396.x) rather than DTCs — but if an inspector finds an active emissions, brake, or lighting code, they may note it as evidence supporting an OOS order.