AutoCodes – Generic – P121B

P121B Code - Cylinder Balance Test Injector Blocked

Quick Answer

The Powertrain Control Module (PCM) monitors the Fuel Injector. Common causes include Faulty Fuel Injector, Fuel Injector harness is open or shorted, Fuel Injector circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P121B indicates Cylinder Balance Test Injector Blocked.
Common causes: Common causes include Faulty Fuel Injector, Fuel Injector harness is open or shorted, Fuel Injector circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 11,259

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Sep 02, 2019

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P121B Description

The Powertrain Control Module (PCM) monitors the Fuel Injector. The PCM sets the OBDII code when the Fuel Injector is not to factory specifications.

OBDII Code P121B - Cylinder Balance Test Injector Blocked - AutoCodes.com — P121B Code - Cylinder Balance Test Injector Blocked

What does this mean?

What are the Possible Causes of the DTC P121B?

Faulty Fuel Injector
Fuel Injector harness is open or shorted
Fuel Injector circuit poor electrical connection

How to Fix the DTC P121B?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P121B code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P121B code?

Need more information on how to fix the P121B code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P121B code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P121B Ford — Cylinder Balance Test Injector Blocked

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P121B Code

1. What are the common symptoms of OBDII code P121B (Cylinder Balance Test Injector Blocked)?

Symptoms may include rough idling, engine misfires, poor acceleration, and decreased fuel efficiency.

2. What can cause the OBDII code P121B to trigger?

This code is typically triggered when the PCM detects a blocked or malfunctioning fuel injector during the cylinder balance test.

3. How can I diagnose the issue related to OBDII code P121B?

Diagnosis usually involves conducting a cylinder balance test, checking fuel injectors for blockages or malfunctions, and inspecting related wiring and connectors.

4. Can a dirty fuel injector cause code P121B to appear?

Yes, a dirty or clogged fuel injector can disrupt fuel delivery and trigger the P121B code due to improper cylinder balance.

5. What are some repair tips for addressing OBDII code P121B?

Repair options may include cleaning or replacing the blocked fuel injector, resolving wiring issues, and performing a cylinder balance test to ensure proper functioning.

6. Is it safe to continue driving with OBDII code P121B present?

It is not recommended to drive with this code active, as it can lead to further engine damage and reduced performance.

7. Can a software update fix the OBDII code P121B issue?

In some cases, a software update provided by the manufacturer may address issues related to the cylinder balance test and injector functionality.

8. Are there any preventive measures to avoid encountering code P121B?

Regular maintenance, such as using high-quality fuel and performing fuel system cleanings, can help prevent fuel injector blockages that may trigger the P121B code.

9. How much does it typically cost to repair OBDII code P121B?

The cost of repair can vary depending on the extent of the issue, but it may involve expenses for diagnosing the problem, cleaning or replacing the fuel injector, and any necessary parts or labor.

10. Can a DIY enthusiast fix OBDII code P121B at home?

While some may attempt to address the issue themselves, diagnosing and repairing code P121B often requires specialized tools and expertise, so seeking professional help is recommended for proper resolution.