“Circuit High” (e.g., P0113, P0123, P0108) and “Circuit Low” (e.g., P0112, P0122, P0107) DTCs describe what the PCM observed on the signal wire — not what failed. The fault could be in the sensor itself, the supply voltage, the ground, the wiring, or a connector. This guide gives you a repeatable workflow to prove which one before replacing anything. It focuses on 3-wire analog sensors (5V reference / ground / signal) but the principles apply broadly.

What circuit high and circuit low actually mean

Tools needed

• Digital multimeter (DMM) set to DC volts, 20V range
• Backprobe pins or breakout leads — non-destructive access to live circuits (see backprobing safely)
• Wiring diagram with pinouts and expected voltages for the circuit
• Scan tool for live data PIDs, freeze frame, and bidirectional control
• Oscilloscope (optional) — needed for waveform integrity checks on rationality faults (see scope basics)

Step-by-step diagnostic checklist (3-wire analog sensors)

1. Verify reference voltage first. Key on, engine off (KOEO). Backprobe the 5V reference pin to the sensor ground pin. Expect a stable 4.9–5.1V. If the reading is low or absent, test the 5V reference bus before going further — a collapsed bus affects every sensor sharing that circuit. See how to test a 5V reference circuit.
2. Check ground quality. Measure voltage drop from the sensor ground pin to battery negative or a known-clean chassis point. Under load, this should be below 0.1–0.2V. A high ground drop causes false highs and lows at the PCM regardless of what the sensor is doing.
3. Measure signal voltage at KOEO. Backprobe the signal pin to ground and compare to the expected baseline for that sensor — typically 0.5–1.5V at key-on, but check service data. Wiggle the harness while watching the reading. A dropout or jump during wiggling points to an intermittent wiring fault.
4. Unplug the sensor and observe what happens to the signal. This single test separates a shorted sensor from a shorted wire:
   • Signal rises to near 5V after unplugging → normal pull-up behaviour. If the code was a low code, the sensor was likely pulling the signal down. Suspect a failed sensor.
   • Signal stays pinned low after unplugging → the signal wire has a short to ground independent of the sensor.
   • Signal stays pinned high after unplugging → open ground wire or short to power on the signal line.
   • Signal floats or reads erratically → open signal wire or poor connection in the harness.
5. Compare signal at the sensor connector versus the PCM pin. If accessible, backprobe the same signal wire at both ends. A meaningful voltage difference between the two points means resistance or a partial fault exists somewhere in the harness between them.
6. Monitor live data if the circuit checks out. If supply, ground, and signal all test good but the DTC persists, you are likely dealing with a rationality or performance fault. Use scan tool PIDs to watch sensor response as conditions change — a MAP sensor should track vacuum, a TPS should sweep smoothly with throttle movement. A scope captures waveform noise and dropouts that PIDs miss. See live data diagnostics and oscilloscope basics.
7. Repair the circuit before replacing the sensor. Fix wiring faults, clean corroded terminals, and repair ground paths first. Retest voltages under load, clear codes, and road test with live data monitoring active.

Fast decision table

Common mistakes to avoid

• Replacing the sensor without checking the circuit first. Most circuit high/low codes are not caused by a failed sensor — they are caused by wiring, grounds, or reference voltage problems.
• Measuring to chassis ground instead of the sensor ground pin. A bad sensor ground creates a voltage offset that makes every reading look wrong. Always measure relative to the sensor’s own ground pin.
• Skipping the unplug test. This is the fastest way to determine whether the fault is in the sensor or the wiring. Don’t skip it.
• Using static voltage checks for rationality codes. A sensor can read a plausible voltage at rest and still fail under changing conditions. Live data and a scope are required for performance and rationality faults.
• Ignoring shared bus faults. One shorted sensor or pinched wire can pull down the 5V reference for every sensor on that branch, generating codes across multiple unrelated systems simultaneously.

2-wire sensors: different wiring, same logic

The workflow above applies to 3-wire sensors with a dedicated 5V supply, a signal, and a ground. Many temperature sensors — engine coolant temperature (ECT), intake air temperature (IAT), transmission fluid temperature, and fuel temperature — use only two wires. Understanding the difference prevents misdiagnosis on these codes.

In a 2-wire NTC thermistor circuit, the PCM applies 5V through an internal pull-up resistor to one wire. The sensor itself is a negative temperature coefficient (NTC) resistor: its resistance falls as temperature rises. The sensor’s second wire goes to the PCM sensor ground. At any given temperature, the sensor resistance and the PCM’s internal pull-up create a voltage divider. The PCM measures the resulting signal voltage and converts it to a temperature reading.

Because there is no external 5V reference supply pin, the unplug test behaves differently than it does on a 3-wire sensor:

• After unplugging a 2-wire sensor, the signal pin will read near 5V. This is the PCM’s internal pull-up with nothing loading it down — it is normal and expected, not evidence of a short to power.
• Circuit high on a 2-wire sensor (e.g., P0113 IAT circuit high, P0118 ECT circuit high) means the PCM is seeing near-5V, which it interprets as extremely cold and high resistance. Causes: open circuit in either wire, disconnected sensor, or a sensor element that has failed open (resistance gone to infinity).
• Circuit low on a 2-wire sensor (e.g., P0112 IAT circuit low, P0117 ECT circuit low) means the signal is being pulled toward ground — a short to ground somewhere in the signal wire, or a sensor element that has failed shorted (resistance near zero).

Diagnostic approach for 2-wire sensors:

1. Measure resistance across the sensor terminals at a known temperature. Service data provides resistance-vs-temperature curves or specific values — a typical ECT reads approximately 2.0–2.5 kOhm at 20°C, dropping to around 200–300 Ohm at 80°C. Zero ohms means the element has shorted; infinite ohms means it has failed open. Either indicates a failed sensor.
2. Check continuity of both wires from the sensor connector back to the PCM. An open in either wire causes a circuit high code without a failed sensor. Use a known-good pin probe at both ends to rule out harness damage.
3. Verify sensor ground quality. Measure voltage drop from the sensor ground pin to battery negative. A high drop (above 0.1–0.2V) causes an artificially low signal reading and can set a circuit low code even with a functional sensor. See how to test engine and chassis grounds.
4. Compare actual temperature to scan tool reading. With the engine at a known temperature — cold soak overnight or at full operating temperature verified with an infrared thermometer — compare the ECT PID to the actual coolant temperature. A significant discrepancy with a plausible circuit reading suggests the sensor element has drifted rather than failed hard.

Platform-specific 5V reference bus architecture

The 5V reference bus is wired differently across manufacturers, and that architecture changes how a single fault cascades into multiple codes. Knowing the platform pattern narrows the search significantly.

• GM: Most GM platforms use two or more isolated 5V reference buses. One PCM pin supplies one group of sensors; a separate pin supplies another. A short on one bus affects only the sensors on that branch, not all sensors simultaneously. Three sensors with circuit low codes that all share a single reference pin at the PCM — identifying which branch is affected from service data narrows the harness trace immediately rather than chasing individual sensors.
• Ford/Lincoln: Many Ford platforms use a single shared 5V reference bus through the PCM. A short anywhere on that bus generates circuit low codes on every sensor sharing the reference — MAF, MAP, TPS, APP, and multiple temperature sensors can all set codes simultaneously from a single pinched wire. Multiple unrelated circuit low codes appearing together after a single event point here first.
• Toyota/Lexus: Toyota typically provides individual 5V references per sensor on modern platforms, which limits cascade failures. However, Toyota uses a shared sensor ground bus (the SG circuit). A poor connection or resistance on the SG ground causes circuit high readings across multiple sensors simultaneously — the ground reference is elevated, which looks like a voltage increase to the PCM. Multiple circuit high codes with a good 5V supply check point to the SG ground circuit.
• Honda/Acura: The PCM chassis ground at the G101 point (location varies by platform but is typically at the bulkhead or near the PCM bracket) is a known failure point on high-mileage Honda vehicles. Corrosion at this single ground connection can affect multiple sensor circuits simultaneously. Circuit codes on sensors that test fine in isolation on a Honda often trace back here.
• VAG (VW/Audi/Skoda/SEAT): VAG platforms commonly show sensor codes appearing in pairs when there is a 5V reference bus fault — a MAP and TPS code together suggest shared reference rather than two independent sensor failures. VCDS provides measuring block values directly in a more readable format than generic OBD-II PIDs, which helps confirm whether the fault is on the sensor signal or the supply bus.

Frequently asked

What is the difference between a circuit high code and a circuit open code?

A circuit high code means the PCM measured a signal voltage above the maximum expected range — not necessarily that the circuit is open. A circuit open is one specific cause of a high reading. Other causes include a shorted signal wire to the power supply, an open sensor ground, or a failed sensor with an internal high-resistance fault. The unplug test (described above) separates these quickly: if the signal stays high after unplugging the sensor, the wire or ground is the fault. If it rises to near 5V after unplugging from a low reading, the sensor was pulling the signal down.

My sensor tests correct on the bench but the code keeps returning. Why?

Bench testing a sensor measures it at room temperature, static, and with no vibration. The fault may only appear at operating temperature (thermal expansion changing contact resistance), under mechanical vibration (loose terminal breaking contact), or under electrical load (the 5V reference bus collapsing under the load of multiple sensors). Test the circuit under the conditions that trigger the fault — at operating temperature, with the harness under road-load flex — not at a static bench. See how to diagnose intermittent faults for the full methodology.

Can a bad ground cause a circuit high code?

Yes. The PCM measures signal voltage relative to the sensor ground. If the sensor ground wire has high resistance or a poor connection, the ground reference is elevated rather than true zero. This makes the signal voltage appear higher than it actually is from the sensor’s perspective, which can set a circuit high code even when the sensor and signal wire are both good. Voltage drop testing across the sensor ground wire under load will confirm this — see voltage drop testing for the procedure.

Why am I getting circuit codes on multiple sensors at the same time?

Multiple simultaneous circuit codes almost always mean a shared cause — either the 5V reference bus is collapsed (affecting every sensor on that branch), a shared sensor ground is faulty (affecting every sensor tied to that ground), or battery voltage is low enough that the reference supply cannot regulate correctly. See how to test a 5V reference circuit and why low voltage causes multiple DTCs before testing individual sensors.

The signal looks correct at the sensor connector but the code keeps setting. What next?

Compare the signal at the sensor connector versus the PCM pin. A voltage difference between the two ends of the signal wire indicates resistance in the wire or a connection fault between the sensor and the PCM. Measure continuity along the wire, inspect any splices and inline connectors, and check that the PCM pin is fully seated and not corroded. A signal that looks correct at the sensor but reads differently at the PCM pin means the fault is in the wiring between those two points, not in the sensor itself.

Related articles

• How to test a 5V reference circuit
• What causes a 5V reference short
• How to use live data to diagnose sensor issues
• Oscilloscope basics for sensor diagnostics
• How to backprobe a connector safely
• How to test a MAP sensor signal
• How to diagnose throttle position sensor faults