P218C: Throttle/Fuel Inhibit B Circuit Range/Performance

P218C means the inhibit B line is electrically within limits, yet its state or response does not fit what the controller expects: it may signal a lock when none is requested, or fail to react during a function test. No manufacturer documents P218C publicly. GM's 2005 table for the 3.5-litre V6 (L66) shows the analogous test on line A as P2553: a function check of the relay path, judged by a normal signal on the serial data link with battery voltage above 7 V, run once per drive cycle after ignition off and lighting the MIL on the next drive. California's OBD rule asks for plausibility checks of such inputs. Likely causes are a sticking relay, a slow-switching driver or software mismatch between the modules. P218D/P218E cover hard low and high faults.

Low severity. Can I keep driving?You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.

Symptoms

Check engine lamp on the next drive
Occasional power reduction without obvious reason

Most likely causes

Relay in the inhibit path sticking or switching late
Software versions of the communicating modules do not match
Intermittent contact causing implausible line states

What to check, in order

Start with the simplest. You can tick off each step.

Check the connector and wiring
Compare the reading with other engine data while varying the engine speed.
Use live data to compare the reading with actual conditions
Measure power, ground, and signal
Replace the sensor if the circuit is good

Good to know

Repair difficulty: hard
It can trigger limp mode.
It affects emissions.

What each character of P218C means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
8C: Specific fault in that system

Frequently asked questions

Can I keep driving with code P218C?

You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.

What causes code P218C?

The most common causes are: relay in the inhibit path sticking or switching late; software versions of the communicating modules do not match; intermittent contact causing implausible line states.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.