P0238 — Turbocharger Boost Pressure Sensor A Circuit High

What Is P0238?

P0238 is stored when the ECU reads a voltage higher than expected from the turbocharger boost pressure (MAP) sensor circuit. A circuit-high fault typically indicates an open circuit in the signal or earth wire, causing the signal wire to float to the reference voltage, or a short to battery voltage on the signal wire.

The MAP sensor uses a 5V reference voltage and outputs a proportional signal between roughly 0.5V and 4.5V. When the signal reads above 4.5–5V, P0238 is triggered. The engine may enter limp mode as the ECU cannot trust the boost pressure reading.

Common Symptoms

Engine management light on
Limp mode with significant power reduction
Boost gauge reading maximum (on vehicles fitted with one)
Poor fuel economy
Flat acceleration
Possible black smoke on diesel

Common Causes

Open circuit in the MAP sensor earth wire — most common cause of a high signal

Open circuit in the signal wire causing it to float to reference voltage

Short to battery voltage on the MAP sensor signal wire

Failed MAP sensor producing incorrect high-voltage output

Corroded sensor connector with high resistance on earth pin

Wiring damage from heat or abrasion near the intake or turbo

How to Diagnose P0238

Check Sensor Earth Connection

A broken earth wire causes the sensor output to read abnormally high. With the connector plugged in and ignition on, back-probe the earth pin at the sensor connector. You should read near 0V. If it reads high, the earth circuit is broken.

Test Reference Voltage

Back-probe the reference voltage pin — should be 5V with ignition on. No voltage means a wiring fault. Correct voltage with a high signal output points to a failed sensor or broken earth.

Check for Short to Battery

With the connector unplugged and ignition on, measure voltage on the signal wire at the ECU end. Constant battery voltage (12V+) on a signal wire indicates a short to the main supply.

Inspect the Connector

Corroded or damaged pins at the MAP sensor connector are common. Clean with contact cleaner. A high-resistance contact on the earth pin gives the same effect as an open circuit.

Replace the MAP Sensor

If reference voltage and earth are confirmed correct but the sensor still outputs a high signal, the sensor element has failed. Replace with an OEM-equivalent unit.

Trace Wiring for Damage

If the fault is in the wiring rather than the sensor, trace the MAP sensor harness from the connector back towards the ECU, looking for chafing, heat damage, or breaks.

Circuit High vs Circuit LowP0238 (circuit high) and P0237 (circuit low) are complementary codes. High usually means open circuit earth or short to supply. Low usually means short to earth. This logic helps narrow the fault to a specific wire before tracing the whole loom.

Verdict

Test the earth wire first — an open earth is the most common cause of P0238. Then check reference voltage. If both are correct, the sensor itself has failed.