P038B

Code

Generic P — Powertrain

Ignition I Control Signal Circuit Low

Brand: Generic

Type: P — Powertrain

Views: UK: 21 EN: 39 RU: 26

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Page language: EN

Causes

Open or shorted wiring in the Ignition I control circuit
Corroded/loose connector at ignition coil(s) or PCM
Faulty ignition coil, ignition module, or coil-on-plug (COP) unit
Failed PCM/ECM ignition driver transistor
Poor battery/charging system voltage or engine ground
Intermittent connector contact from vibration or heat damage

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Rough idle, misfire, or specific cylinder misfire code(s) logged
Hard start or no-start condition (if control signal absent)
Loss of power or poor acceleration
Intermittent engine stalling or reduced fuel economy

What to check

Read and record DTCs and freeze-frame data; note related misfire or cylinder-specific codes
Visually inspect ignition coil(s), connector(s), wiring harness, and PCM connector for corrosion, heat damage, or pin push-out
Check battery and charging system voltage under load; verify good engine/chassis grounds
Backprobing: measure voltage at the Ignition I control pin at the coil connector while cranking and idling
Compare signal at coil connector to PCM output pin to determine if fault is in wiring or PCM
Inspect/verify fuses and relays that feed ignition circuits (if applicable)

Signal parameters

Inactive (coil driver OFF): ≈ battery voltage (11–14.5 V) at coil control terminal
Active (coil driver ON, low-side driver): pulsed near 0–1 V when driver sinks current
Pulse frequency: engine rpm-dependent; one pulse per ignition event for that coil (up to several hundred Hz for high RPM and multiplexed coils)
Pulse width (dwell): typically 1–6 ms at idle, varies with load and engine management strategy
Expected waveform: sharp square pulses when viewed with an oscilloscope; missing pulses or flat low voltage indicates a fault

Diagnostic algorithm

Record all stored DTCs and freeze-frame; note which cylinder(s) are affected and any related codes (misfire codes P030x, P035x).
Visually inspect the coil(s), wiring harness, and connectors for damage, corrosion, melted insulation, or oil contamination. Repair as needed.
Verify battery voltage (≥11.5 V) and good engine grounds before performing signal tests.
With ignition OFF, disconnect connector and check coil power and ground terminals for continuity to battery and chassis ground. Repair any open/poor connections.
Backprobe the Ignition I control terminal at the coil with a multimeter/oscilloscope. Crank and run engine: verify pulses go from ≈ battery voltage to near 0 V (for low-side drivers) at the expected timing. If pulses are present at coil, suspect coil/module. If no pulses, continue testing toward PCM.
Backprobe the PCM ignition output pin and compare waveform/voltage to the coil-side measurement; if PCM output is normal but coil-side is low, suspect wiring short to ground between PCM and coil or poor connector. If both are low/absent, suspect PCM driver fault.
Perform resistance check between coil control pin and PCM output pin with ignition OFF; look for short to ground or opens. Wiggle harness while monitoring to find intermittent faults.
If available, substitute a known-good coil or coil module to see if code and symptoms clear (use manufacturer-specified replacement).
Repair wiring/connectors or replace failed coil/module. If testing indicates PCM driver failure and wiring is confirmed good, consult manufacturer resources before replacing PCM.
Clear codes and road-test to verify repair; re-scan for any reoccurrence.
Safety note: take care when probing ignition systems; use appropriate insulated tools and follow vehicle service procedures to avoid shock and damage.

Likely causes

Short to ground on the ignition control lead (causes low voltage)
Open/high-resistance connection between PCM and coil (loss of drive signal)
Faulty ignition coil or coil module that loads the driver
Corroded or damaged connector pin at coil or PCM
PCM driver output fault

Fault status

⚠️ Status

PCM reports Ignition I control circuit voltage below expected threshold or missing pulsed drive. The engine controller cannot properly energize the ignition coil(s) on the Ignition I circuit, which can cause misfires or no-start conditions. Inspection of coil, wiring/connectors, and PCM output is required.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours